As general rule is not a good idea to install different versions of the plugin, because the libraries, maps and materials are stored in a same folder at c:/program files. This may cause incompatibility issues (as FP 4.0 using the libraries of FP 4.2).
The right procedure should be to install FP 4.2.4 for Max 2014, and next repeat the installation for Max 2015. In this case the setup program will not remove the 2014 files.
We are working to support multiple Max versions in the installer.