The PC will be technologically ahead of the PS4 (it already is). However, that's not relevant, as games are made for the consoles and then ported to PCs. My computer is from 2009 and still runs almost anything on maximum settings. I didn't upgrade it nor even reinstall the OS or anything, since I bought it.
Maybe with the PS4 and X720 the games may take a leap forward (although not as big as in previous generations, publishers can't keep escalating development costs) and force me to upgrade to keep up. But except for indies, I can't come up with a name of an AAA game exclusive to PC or made primarily for PC (Maybe The Witcher 2?). SO as long as consoles keep being the primary focus, there's no need to worry about buying a new computer or to upgrade it.