I have a 20 month gap between my youngest two - both adopted and placed 20 months apart. Also have another adopted - now adult - child. My three arrived separately and all were under 12 months old.
I would be very wary. At the risk of sounding cynical, you may not get the support you need post placement from SS - that’s me being diplomatic!
Apart from the emotional, trauma caused issues think about practical ones. Sharing toys, belongings etc. Presume separate rooms? School can throw up many difficulties and your birth son won’t be able to escape them unless they’re in different schools. They’ll be two school years apart? That certainly caused issues for my eldest son when middle son experienced difficulties in school. We ended up with three in three schools.
You say both boys are attention seeking? Attention needing. How would you manage competing demands? A 4 year old isn’t going to be napping to give the older one space? At least with a younger child you will be better able to give separate attention to each.
There will be jealousy - your son is used to your undivided attention. Competing with a baby is one thing. Competing with a vocal, mobile, attention seeking 4 year is something else.
Have you spoken to his foster carer? His school? Do you have a full history?
My three get along well now - 20, 17 and 16 - but it’s been a rocky road at times and, frankly, a full time job. Assume he’ll have additional needs - because he will. How will you accommodate those? I guess at least SS will be able to share some of those needs with you .. they hadn’t been identified in my younger two.