That's a real good lookin' set-up.
With more-or-less full-sized coops, we don't usually need to be concerned about cubic feet of air. However, structures of this size require a look at more than just floor space. That said, with a mild climate and the chance to get out and under - an "extra" bird isn't likely to cause any suffering.
The rule of thumb is that 3 cubic feet is needed for every pound of chicken in permanent indoor housing. With a ceiling high enuf for humans and without caging and the stacking of cages, there's usually more reason to be concerned about floor space for the birds. With your low, low ceiling, closing up this little house in bad weather could turn it into a very unhealthy place if there were very many birds in there.
At 3' x 4', they've got 12 square feet of floor space. If the average height is 3.5', the birds only have 42 cubic feet. Divide 42 by 3 and the rule of thumb says that you've only enuf room for 14 pounds of chickens.
If they are 4.5 pound leghorns, there's room for 3. Of course, if they are not really closed in, there isn't as much concern.
Steve