8x4=32 sq feet. The height of the walls doesn't really matter, it's floor area you're interested in.
I'll give you the general rule of thumb for what is considered minimum space per bird, it's 4 sq ft inside the coop. According to that, your coop would be best suited for 8. Not to say 10 won't fit because they will. Will there be available space for them to get in and out? Unless you're locking them in the coop for days at a time, they're basically just sleeping in the coop at night.
If you leave the food and water in there, with open access to get in and out all day to eat and drink, it will still be a rare occurance to have them ALL in there at the same time trying to eat. I would say you'll probably be ok with 10, unless there will be times you'll have them stuck inside for extended periods of time. They get cranky if they're overcrowded