You will want to slowly introduce them. Sometimes male rats, much less adult rats of any gender may not be as welcoming to new cage mates.
Make sure your cage is plenty big for 4 rats.
Be careful of mixing young and older, sometimes this just isn't a good idea, but again it will vary on the rats, and how you introduced them.
A good idea is to set up a separate cage next to the main cage that you want to use. Put the 2 new rats in the new cage. Letting the older rats sniff them. Leave this setup for a few weeks. This will let the rats get used to each other and it will help ensure that the new rats aren't sick. (Generally, it's actually best to make sure they're not sick before even putting the cages together.) Once they seem ok, let them play outside the cage together, and slowly introduce them into the same cage, with short periods at a time.