My prediction is that 4v4 matchmaker won't take off because people will experience lag/slowdown from slow computers and lag, especially on 20x20 maps.
It might be best to start with only 10x10 maps for the rollout and see what happens. Then put some smaller 20x20s in the pool to see if bad things happen. I would suggest starting this way because you can't "unring the bell." If people try TMM and have disappointing experiences, they might not come back. But if the rollout starts successful, you could build on that.