Update (07/01/21): Fleshed out the later age possibilities. Apart from trying to get FE+ troops in PME you should generally not let map or tech get ahead of story, and save as many optional techs as you can.
1) Iron Age - Obtainable (2) Armored Infantry : If you complete the story up to the point where you unlock the EMA bonus area (barbarians), and finish the bonus questline, the final reward is 2 armored infantry. How much does this help? Well it'll make fighting through difficulty 3 a breeze - but 4 might still need significant boost and care not to lose your precious unattached units.
2) Late Middle Ages - Obtainable Grenadiers, Musketeers, Field Guns : If you complete the LMA story it unlocks the LMA bonus area to the south. At various points along the way this questline rewards Colonial Age troops. To finish it entirely you will need to research the tech for Cannons - but some of the units are available very early. How much does this help? Some... I'm not sure it's enough to fight 4 without losses so as with iron age some care may be required not to lose your precious unattached units. I've heard someone say they got Jaeger Infantry (Industrial Age) in LMA and that would make things a breeze - but I can't see how that's possible as Colonial Age story has quests that require construction of Colonial Age buildings. Only thing I can imagine is unlocking the Industrial Age bonus area but as far as I know those don't reward units. (Note if trying that, you cannot finish the Industrial Age bonus questline and get that quest slot available for other quests again without Modern Roads (Industrial Age))
3) Colonial Age - Obtainable All Industrial Troops, Conscripts OR 1 Rapid Fire Cannon + 2 Snipers + 1 Armored Car. The Industrial Age troops are relatively straight forward to do and enough to fight most of GE4 without difficulty combined with rogues. The progressive era troops you have to enter Colonial age with it in mind that you're going for them. There are a number of quests that will request things like 'Build a Residential building from the Industrial Age OR Research a tech that unlocks a residential building' so you'll need to save as many techs as you can to make sure you can fulfill that option. In doing the Industrial Story, you should note there are 4 ways it can progress: Support the South, Support the North, Fight both, Peace. I recommend supporting the South (fighting only the northern provinces). The combat might be a little more challenging but you can complete the story without unlocking the bonus questline and maintain your extra sidequest slot til you're ready. It also gives you a nice mix of all Industrial troops (I can't say for sure what troops exactly you get in the other route). When it comes to Progressive Era, you again have the same sort of options. Peace gives you conscripts. Fighting the germans gives you the RFC+snipers+car. I believe fighting the brits gives the same troops. The good news : either progressive era troop option should be plenty to breeze through GE4.
4) Industrial Age - Obtainable PE, ME, PME troops - Most notable is the MG team from PME. To get these it's important to stay at peace with Che Guerrero, and not to finish his last territory for as long as possible. This will give you "Business with Che" quests which will reward you with MG teams.
5) Postmodern Era - You can get a few CE troops "normally". There's also a method to skip the CE blocker quest in PME and get to the advanced troops listed under CE... I've not done it myself to recommend it but the gist is that *before* finishing PME story quests you skip way ahead on the continent map, working your way through to FE map. Once on FE map you start catching up on story and it'll bypass the CE blocker hopefully. There are more details on this elsewhere - look around if you're interested :)
6) Contemporary Era - If you've skipped enough technologies, you can get through to OF quests here. FE Hover Tanks are perhaps the most notable troop at this point - to get 4 Hover Tanks instead of 1 you should negotiate with Joy Danba at the start of FE story - then when you meet her again later in the map she'll give you 3 Hover Tanks. An OF quest requiring you to finish productions in an OF production building acts as a final block preventing you from getting any further troops above your era.
1) Iron Age - Obtainable (2) Armored Infantry : If you complete the story up to the point where you unlock the EMA bonus area (barbarians), and finish the bonus questline, the final reward is 2 armored infantry. How much does this help? Well it'll make fighting through difficulty 3 a breeze - but 4 might still need significant boost and care not to lose your precious unattached units.
2) Late Middle Ages - Obtainable Grenadiers, Musketeers, Field Guns : If you complete the LMA story it unlocks the LMA bonus area to the south. At various points along the way this questline rewards Colonial Age troops. To finish it entirely you will need to research the tech for Cannons - but some of the units are available very early. How much does this help? Some... I'm not sure it's enough to fight 4 without losses so as with iron age some care may be required not to lose your precious unattached units. I've heard someone say they got Jaeger Infantry (Industrial Age) in LMA and that would make things a breeze - but I can't see how that's possible as Colonial Age story has quests that require construction of Colonial Age buildings. Only thing I can imagine is unlocking the Industrial Age bonus area but as far as I know those don't reward units. (Note if trying that, you cannot finish the Industrial Age bonus questline and get that quest slot available for other quests again without Modern Roads (Industrial Age))
3) Colonial Age - Obtainable All Industrial Troops, Conscripts OR 1 Rapid Fire Cannon + 2 Snipers + 1 Armored Car. The Industrial Age troops are relatively straight forward to do and enough to fight most of GE4 without difficulty combined with rogues. The progressive era troops you have to enter Colonial age with it in mind that you're going for them. There are a number of quests that will request things like 'Build a Residential building from the Industrial Age OR Research a tech that unlocks a residential building' so you'll need to save as many techs as you can to make sure you can fulfill that option. In doing the Industrial Story, you should note there are 4 ways it can progress: Support the South, Support the North, Fight both, Peace. I recommend supporting the South (fighting only the northern provinces). The combat might be a little more challenging but you can complete the story without unlocking the bonus questline and maintain your extra sidequest slot til you're ready. It also gives you a nice mix of all Industrial troops (I can't say for sure what troops exactly you get in the other route). When it comes to Progressive Era, you again have the same sort of options. Peace gives you conscripts. Fighting the germans gives you the RFC+snipers+car. I believe fighting the brits gives the same troops. The good news : either progressive era troop option should be plenty to breeze through GE4.
4) Industrial Age - Obtainable PE, ME, PME troops - Most notable is the MG team from PME. To get these it's important to stay at peace with Che Guerrero, and not to finish his last territory for as long as possible. This will give you "Business with Che" quests which will reward you with MG teams.
5) Postmodern Era - You can get a few CE troops "normally". There's also a method to skip the CE blocker quest in PME and get to the advanced troops listed under CE... I've not done it myself to recommend it but the gist is that *before* finishing PME story quests you skip way ahead on the continent map, working your way through to FE map. Once on FE map you start catching up on story and it'll bypass the CE blocker hopefully. There are more details on this elsewhere - look around if you're interested :)
6) Contemporary Era - If you've skipped enough technologies, you can get through to OF quests here. FE Hover Tanks are perhaps the most notable troop at this point - to get 4 Hover Tanks instead of 1 you should negotiate with Joy Danba at the start of FE story - then when you meet her again later in the map she'll give you 3 Hover Tanks. An OF quest requiring you to finish productions in an OF production building acts as a final block preventing you from getting any further troops above your era.
7) Tomorrow Era - You can certainly finish whatever you didn't in CE here in getting to that OF blockade. OF Sub Cruisers I find quite useful in TE.
