The purpose of 4 bolt mains is not to keep the cap together because it's fine. The purpose is to spread the load out in the block to keep the bolt holes in the block from breaking.
I had a 316 cubic inch drag race engine years back that was built using a 283 2 bolt block (they all are). It made about 525 hp and was spun to 8000+ rpm. It ran fine with no troubles and I ran it quite lot for about half a seaon. I tore it down to hone and re-ring it, as was normal practice. While unloading the block out of my pickup, my engine builder's eagle eye noticed a crack adjacent to one of the main cap bolt holes. Turned out that the main webs were cracked at the bolt holes on all 3 center caps, 12 places in all (fore and aft on each bolt). So much for honing that block, I ended up getting another block machined and put aftermarket 4 bolt caps on it. It ran perfectly after that, and I had 4 bolt mains on every race engine I built or used after that.
I think it's the rpm that kills them, not the power. But it really doesn't matter because power and rpm go hand in hand.
So I would recommend 4 bolt mains for any small block that turns over 6500-7000 rpm regularly. I've never had a big block performance engine that didn't have 4 bolt mains, but I think the same guideline would apply.
And you're right Otis, spinning bearings generally has to do with oiling, having the proper crush on the bearings, and the proper bearing clearance and crankshaft finish. But if you run an engine with cracked bolt bosses, you are losing crush, and that could lead to a spun bearing. In my case the bearings were still fine because it was caught soon enough.