The vita is not doing very well, though that is something for an entirely different topic, and calling it Playstation 4 would keep in line with previous consoles and it sounds simple, classy - and it wouldn't confuse the general consumer... Although theres the odd chance they just might call it "The Playstation", like microsoft is rumored to do with their next console (ie, calling it "The Xbox").
I have both a Playstation, Playstation 2 and a Playstation 3.
Would like to get a Playstation 4 too when that comes out.
Provided it's as awesome as I hope.