Yep, I have mine in a garden shed 8x6ft - useful to have a stable door and look for pent roofs. Just amend the windows to have mesh instead of glass and it's perfect. Much easier to clean out and they appreciate the extra neck room, I think. I've got 3 adult geese and 9 younger ones at the moment so I'll see whether they'll all manage to fit. I think it'll be tight but I have buyers for three already so I don't imagine they'll be that crowded for long. The eventual aim is to have two trios so six of them would have to share it out of breeding season and that would be the same space per bird as your three in a 6x4.