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?