An 85% efficient 850W would give ~722W to play with, minus 70W for system leaves ~652W for GPUs. ~652W/4 = 163W per GPU available.
That's not how PSU efficiency is measured. The rated Wattage is what the PSU can output, not what it will draw from the socket. So, an 850W PSU will actually give you ~800-850W to play with, depending on the rail configuration. Again, the important one is the 12V rail, which in a typical quality 850W unit will deliver 70A or thereabout, so ~840W.
You are correct this is not how PSU efficiency is measured, however you are incorrect about getting 70A out of a 12V rail on a 850W PSU....unless of course you don't have a motherboard attached to that PSU. If you want 70A out of a 12V rail and intend to use a motherboard and other peripherals such as a CPU/RAM/drive/etc you will need ~1KW depending on other components used.