Welcome! I also adopted a then 4 year old, while having a 1.5 year old since he was a 'teenager'. My advice would be to watch them like hawks for the first little while. The younger one may exhibit jealousy, as mine did, and mope around or want to be aggressive with the other. That's when you step in and stop the behaviour. Watch for precursors of their next move and stop it if it looks menacing.
Let it be your goal to let them both know that they're stuck together and neither one is leaving (lol), and make them play team games. For instance, tug of war is a fave at my house. Walking as much together, and beside each other, was also done daily. Encourage good behaviour with excitement, no matter how small it seems. Give both equal attention and loving.
Good luck! It may take a couple of weeks until you see improvement, but hang in there. One day, they'll surprise you with how well they get along!