I had problems keeping 4 WW barbs in sync when I was testing it on the PTR. The issue is keeping the barbs generating the same amount of fury and keeping fury generation high enough to WW non stop. If you ever don't generate enough fury you stop whirling which leads to desync and then if you aren't spending it fast enough you can't keep Berserker up permanently on all characers which due to its movespeed buff also leads to desync.

Now I was testing barbs without the BK weapon set. If you had that on all 4 characters, keeping fury up enough to constant WW should be much easier and the problem may solve itself. Still getting getting 2 good BK weapons for each character isn't going to be easy (you would need 8 of them for 4 boxing).