will they use the top one? It is about 2 ft, 8 in from the base?
This will probably be the favorite. The highest place they can sleep usually is the favorite.
The other side. I'm thinking I need to add a support to get the bar a bit higher than the boxes (or is that height ok since the boxes are a bit lower?
I don't know how long that top roost is. There is a good chance that is the only one they will use if you only have four chickens. I'm not sure what is going on over the nests either. If they can get on that board that would be the second most popular location, right over the open nests.
I'd think the top of those nest dividers would be the next favorite places. Your two bottom roosts would be least favorite. The top of the boards forming the lip of your nests is the same height so they would be equally popular as your current lowest roosts..