In that case, assuming both Win8 and Win7 are 64bit versions you may have to increase the Ram in your system.
Win8 64bit requires 2GB min to run (supposedly) but in reality you'll probably need 3GB min for efficiency.
Win7 64bit requires 2GB min to run (this can be achieved but may lag on some intensive programs).
4GB in you system may not be enough to keep the Microsuck Host and Guest running in VBox. It would work better if you had Linux as Host and Windows as Guest due to way Linux handles memory.
So you may have very little choice other than increasing to 6GB min.
I think i will switch to Vmware Workstation 
