[RatCrazyGirl]

Could I keep a 2-3 month old rat with a one month old rat as its only cagemate?

 

[Little rat beans]

As long as you do introductions properly and they get along well I think it should be okay. Sometimes it's the only option even with a slight age gap.

 

[CorbinDallasMyMan]

I would not separate a 4 week old rat from it's litter mates. If you want to do this, I would suggest getting 2 young rats from the same litter to introduce to the solo 8-12 week old rat.