Exactly my guess too. There's no way they make a Mac Pro that can make a 4k display look good without making one themselves. My guess is after Christmas though, maybe a Spring event of some sort.
Given that Samsung hasn’t shipped any 4k displays so far, I suspect Apple would rather go with a manufacturer that has experience building them and with whom they don’t have such a complicated relationship. I’m thinking of 4k panel makers like AUO, Innolux, Sharp, and LG.
