When you get such an error, this means that you generally need to recompile your program.
Alternatively, you may install the old MPFR 3.x version (which corresponds to libmpfr.so.4) if available, otherwise from an earlier Ubuntu release (I do not recommend that, but if there are no conflicts from the packaging system, this could be OK; FYI, in Debian, both packages libmpfr4 and libmpfr6 are co-installable).
Note: Contrary to what has been said in the comments, libmpfr.so.4 is no longer there if you're using the 64-bit ABI (x86_64), as ldconfig -p | grep mpfr gave for this file: /usr/lib/i386-linux-gnu/libmpfr.so.4, i.e. the 32-bit version. I suspect that the upgrade removed the x86_64 one (either due to a conflict or because it was no longer needed).