Asus m4a79 deluxe quick question

No. That's why 4 of the 8 pins are covered when the board ships. If you happen to have a PSU with an 8-pin, you can remove the tab and use that, otherwise 4-pin works fine.

do they do that because the "old" psus have the 4 pin and the new ones have the 8 pin?

whats the difference?

There doesn't seem to be any practical difference. I'm sure the ATX spec was changed for a good reason, perhaps because they were expecting processors to pull more current.

Yes, older design PSUs only use the 4-pin.