This is just my personal opinion, so don't feel like I'm telling you NO NO NO.
I have 4 kids ages 8,9,10 & 11 and I would not add a similar/same age to the mix. They are extremely competitive with each other and the fight for alone time or special attention from me or dh is unbelievable at times.
It's not just sibling rivalry, but rather a fight to the death for EVERYTHING. Over toothbrush placement in the holder, where to sit in the car, who got one extra corn flake than another, and even the order they go upstairs to bed. I didn't allow our older two to ride their bikes out of our culdesac until they were 10 due to individual reasons and also them not having had as much time with us at earlier ages. I allow my 9 & 8 year old to do this because they have shown safety and are able to follow the rules. I hear almost daily..."You didn't let me do xx when I was that age, that's NOT FAIR".
Because they are all one grade apart in school, it makes for a very very busy schedule. I'm a SAHM and there are days when I need to carpool or very creatively arrange their schedules in order to get everything done. Same age groups (i.e. 9/10 are paired together in a lot of sports and my kids do NOT want to be on the same team. lol. I don't want them on the same team or competing against each other either because it really causes a lot of issues.
I'm not saying it can't be done. I'm just saying that until you really live the life with 4 kids that close together, you really truly don't have any idea how it is in reality. I can say this because I had thoughts of "Oh, it's no big deal" when we adopted them. Well....lol...it certainly does present a lot of different factors I hadn't considered or realized.
Most importantly...once you finalize, you will still likely have another couple of years before things get to the level I'm experiencing now. Things were actually easier for me when they were little. Plus, just a thought..until things are really stable with your kids, I really wouldn't advise bringing in more children regardless of their ages. Personally I think it takes an older child to really understand "We are fostering, I am STAYING HERE FOREVER even if the other child leaves".
Sorry to be a downer, I really don't mean to be. Overall, our life is great and the kids are a joy. Truly, they are. Just not a life without chaos.