Basically, it's a non-issue. If you've got
a system with more than 4GB of actual RAM, then it must
be a 64-bit-capable system. (Otherwise, you were very-seriously ripped off by a smooth-talking hardware salesman!
Therefore, you should compile that application in the appropriate (i.e.
64-bit) mode, and be done with it.
If you do not have access to the source-code of the application, then its (32 vs.
64 bit) memory limitations "are what they are."