epic patch failure here as well.
After applying patch to MAIN folder got the "interface.MPQ" not found error.

* applying the patch for the second time - same effect.
* repair.exe gave me the finger ans said reinstall game.

* Deleted DATA folder and copied the unpatched Data folder ALT (yes I have clones of the game on my SSD
* Applying the patch, downloading again 1.5G
* start and worked. *sigh*

(another possibility would have been to do a clean install of version 4.0.1 (downloadable from battlenet)