Some people say Sony would not let MS have the more powerful hardware, but i don't see why not. Look at PS360, sure the PS3 was slightly more powerful but it also had to cope with slightly lesser multiplatform games, but despite all that the PS3 sold pretty damn well. I really doubt the userbase of PS4 will make Xbox their main system anytime soon, so i think they will be ok.
I don't think it's a question of Sony letting them, they don't really have much of a choice in the short term if the rumors are true.