Why Not Use All Four Die?
There are several reasons AMD isn’t using four 2+2 configurations. First, 2+2 would increase latency and hit gaming performance, as data has to be passed across the Infinity Fabric. So far, tests have shown that configuring Ryzen 7 1800X in a 2+2 or 4+0 configuration has a fairly limited impact on performance, but that could change when core counts rise to the level Threadripper offers.
The other important thing to know about Threadripper is that the other two die apparently aren’t actually chips at all. They’re apparently structural inserts required for support, but do not represent “bad” Epyc cores or any kind of core recycling. OC3D also states that the two active die in Threadripper are on the diagonal from each other.
Makes sense since AMD is using the same socket design, cheaper than changing to a whole new manufacturing line, just changes the robot picker to pull from another bin.