When should I start training at Oblivion 4?

If you can 5hko 100% of the time, O4 > skeles. For me, this was at 136 with 12 blessing. For most it's like 140ish? If you 5hko and get killed a lot, wait at skeles for a little while longer. If you have a mage to train with (which i know you don't), skeles is always better than O4.

I still have no idea how the pineapple you can 5 hit 100% of the time when I go from 4-6 hits. Maybe that's because I don't use attack pots.
But yeah, O4 is always better than solo skeles, since you get PoT's there. I can DS-Blast-Blast and still make money off the PoT's.