I'll double check tonight in some testing, but I don't believe there's any reason to complete the U3,4 combo, ever. You can make some interesting combo's in the corner with it, and it's possible to link a combo off of U3,4,DF1, but it requires meter and isn't worth it. If you're connecting with U3, you can complete into U3 U3,DF1 xx 3,DF1- xx 2,3,DF3,4 or 1,1,2.
You can dash into U3. You can't dash into U3,4. It used to be that you couldn't dash into U3, but that was a bug and was fixed. U3,4 isn't intended to be able to be dashed into.