How realistic is a 4 ports charging only USB hub mount inside a wall outlet box?
Entirely viable. 
Eben if you had 2A x 4 ports x 5V = 40 Watts you should be able to get 80% efficiency or 8 watts dissipation. 8 Watts (or even 20W) is able to be dissipated either into air space around box or, if needed, via a metal faceplate. 2A x 4 would be extreme probably and you'd usually have much less heat to deal with.