It's odd that you could do so with the 32-bit LR4 and not with the 64-bit LR4. You might see if you may be able to set the file extension to be opened by the 64-bit LR4 in
Default Programs. If it works, then hopefully it should be listed in Open with as well.
Just to be sure, are you expanding the
Other Programs arrow in "Open with" to see if the 64-bit LR4 may be listed there instead of just in the
Recommended Programs section at the top?