PS4 doesn't really have it's Halo, if Microsoft release a Halo 1-4 collection as rumored you can bet your butt people will pick up the 399 xbox one just to play that. Sony doesn't really have anything that's remotely close to Halo right now. Even though I think a megaton game like The Last Guardian could swing it to Sony for the Holidays because people like me who already own an xbox one would pretty much buy a PS4 on principle I suspect it will have limited appeal outside those who visit sites like IGN. I could be entirely wrong though.
Halo doesn't have the same mindshare it used to. It sells, and sells well, but it's more an entrenched franchise that fans return to, than something to convert people on principle. The announcements of Halo 5 (formally, anyway) and rumors of the collection aren't making ripples anywhere but established fandoms, from what we are seeing at retail. Sony doesn't need a single title like Halo, they've always sold on a much larger range of titles, but this generation the big Multi-plats that the 360 was the primary on are shifted to PS4 in preference.