If you did the 4' I think you could add three, since its going to be a 10' long coop right?
I would keep all roosts the same height, so you don't have fighting over who's on the higher one. Also you've got the ventilation, you don't want them too close to it and you also don't want the chicken's...
I have my coops raised off the ground about 3ft I think? Its waist high for me and I'm 5'3".
Here's a link to the coop articles, you should be able to find something to help you.
https://www.backyardchickens.com/articles/categories/chicken-coops.12/
Make sure roosts are always higher than...
An 8x10' run would give 8 birds 10sq ft of space each, and that's the bare minimum.
That coop would be good for 8 birds.
I think it would be a good idea to add more space, you won't regret it! Plus more space the better.
No, if its raised off the ground the space underneath counts.
Adding...
Okay! Well, at a minimum each bird needs 4sq ft of coop space and 10sq ft of run space, and more the better. With a 12x12' run, if you had, lets say 7 birds, each one would have 20sq ft of run space. So a 12x12' run is perfect for the amount of birds you want. Also, since you have 3 bantams...