I expect it to be announced at Playstation Day next year like the Vita was last year. Release at the end of the year.

that would be a nice expensive launch. i wouldn't have to pay so much for the console then but more for games as i had for ps3 haha

Most of the games announced for 2013 are launching in the first half of the year. Not a lot of stuff has been definitively dated for that holiday season, which leaves it open for a new console.

Maybe Watch Dogs can be a launch title.

EDIT: Also, if they add PS3 backwards compatiblity, the strong PS3 lineup can work in their favour.

The new Playstation has to have a 599$ price tag. It's a sony, and they live well with this strategy. The PS3 was expensive, had no gaemz, and was massively subventioned. But it was one hell of a machine. Just think of that: the very same console you bought yeeears ago will be able to play the last of us which will drop your jaw.
And even with a new PS4 out, the PS3 will sell well for anoter 3 years, then decline fast. But that is the time in the 10-year lifetime when it makes constant profit (we are in this phase for about 2 years now i think), which can be used to subvention the new PS4.

And just think about it, Sony has many first party developers whose projects are unknown. they are all experimenting with early development kits, that's for sure. if sony manages to bring the PS4 in the same year as XBox and starts with 3-5 of it's exclusive IPs, namely GT6, Uncharted 4, Infamous 3, GOW perhaps a new spinoff series with new character, maybe new IPs.

And after i've seen more of Nintys Wiiiuuuuuwiiiiiiuuuuu i don't consider that as a real threat for next gen. ninty does kind of the same strategy as for wii. even if you'll now be able playing multiplatform titles on your wiiiiuuuuwiiiiiuuuu. but not for long, graphical differences from wiiiiuuuu to next gen hdhd-twins will be significant from the beginning and increase everytime a new "state of the art"-defining game comes out on the new consoles.

but that's just my two cents and i guess i am OT.

i would like to read the opinion of someone who said 2015. that sounds really late, i wonder why you think that?

Since Last of Us and GOW:A are PS3 2013 titles, Id say 2014.