The "File Mismatch 004" message indicates that there is corruption at the end of your incomplete file.
To get around this you must truncate that portion of the file which is corrupt.
I've found a solution which accomplishes this (100% success rate so far).
1) Cancel the corrupt file
2) Enter a WinMX chatroom
3) Browse yourself
4) Manually download the corrupt file to a new file name
5) When you see yourself in your queue start the transfer
6) Once the transfer has started go to [Settings - Bandwidth Throttle] uncheck the "Limit outgoing bandwidth" option (you should see a transfer rate of around 1,000 K/s)
7) Cancel the transfer before it completes (I cancel about 5Mb before completion)
