I don't recommend making any changes at all to mojoPortal code.
Your reason doesn't really make sense to me because even if you are trying to reference our business layer from another project or assembly that is implemented in .NET 4 you still do not need our assembly to be a .NET 4 assembly. You should never add any custom code into our projects.
If you make that change you will never be able to upgrade to new releases of mojoPortal other than by producing your own build from source code, you will never be able to upgrade using our release packages (at least until we drop support for .NET 3.5 and that will probably be quite a while yet).
My advice is don't do it.