Apparently, that's not a safe assumption. The IP that is the basis for "Stage 3" (the area containing LMA and Catastrophe Canyon) might very well end up being Cars but that isn't set yet and there are reportedly some folks involved in the decision who would prefer to leave that as a DCA exclusive.
As for "Phase 4", Martin's map indicated it would in the the vicinity of the Animation Bldg and backstage around there. IMHO, I think that they'd be using non-Pixar properties in that area.
And there's a large number of people on these forums who would prefer Cars stay a DCA exclusive....
..... We already have test track.