Yes the 4 at the back are the J class loo's, they tend to be more "protective" of the front loo for F pax when F is full and the showers are in use. Having said that when I have traveled in F I notice people coming from J all the time and using the front loo. The crew have usually turned a blind eye and are always polite at least when I have seen them. In my view they should tell people to go to the back albeit in a polite way as that loo is clearly intended for F only. Most of the J pax know this and are just trying it on!