I have 2 of these psu-s running 4 S3 overclocked with no problem.
They can handle 5 for sure. I wasn't able to connect 5 because I've used them for S2-s and didn't soldered enough cables on them.
So you have 4200Watts running 4x S3 units?
or 8 units 4 on each one?
4 on each one.