4GB is overkill for a VM. When I first started using Fusion, I thought you needed to allocate RAM like it was a physical PC. but it can run fine with much less ram. XP for example (which is the only version of Windows I would use) runs fine with 512MB.
That's more then what physical XP needs ...
I use bootcamp for gaming, however I use parallels on all other occasions. 4GB RAM isn't overkill, it is easy to use over 4GB when running many programs on both OSes. 8GB I nice but not necessary for
Most people. OP for your uses 4GB is fine.