Sub encloser question

I have a friend who has 4 10.5lf 4ohm Subs in a custom box.  It looks like they are wired in series from one sub to another. When they hit the first sub looks like its working alot harder then the others. He is using 2 amps Rockford 1001bd amps. Question is should he make a bigger box and should they be wired like they are? The box he has now looks like its fits specs but it just seems too small. Thanks for the help!

Are you saying that he wired four 4Ohm voice coils in series? If so, that means that the ovreall Ohm load is ~16ohm nominal. That is quite high. I would suggest that he wire two of the voice coils in series, then seperately wire the other two coils in series and then connect both sets to the amplifier in series. This would give you a 4ohm nominal load. As for the box not being big enough.. if it's built to the manufacture's specs, then that's good enough