Is this ok
4x S3+ OC'd
You need to get the notion that you need 4 connectors populated to power an overclocked S3+ out of your mind.
I'll do some maths for you to judge what you need.
Each 6 pin PCI-e connector, fully populated with PROPER 16AWG cables carries 7.5A * 3 = 22.5A which equates to 270 Watts @ 12V
That is enough to power a board in an overclocked S3+ (just one socket!), so for a 1200W PSU, you can safely power 2 S3+'s overclocked. Having said that, a single S3+ will not draw that much power, so a 1000W PSU can safely power 2 overclocked S3+'s.
Depending on how much power is available on each rail of your PSU, one 6 pin PCI-e socket should be adequate per S3+ board (making it 2 for the rig), but if you chose to populate all four, go ahead. For the cost and headache of an off the shelf PSU, I'd recomend a server PSU which are fare more efficient and robust than their desktop counterparts (and cost next to nothing, again compared to their desktop counterparts!)