The best thing you can do is add more RAM to the Host and use a 64-bit Host OS! 
Also even though you've assigned 512 MB RAM to the VM an additional 128 MB is used for video (256 MB with a Windows 7 VM). So if you take 640 MB physical RAM for each VM x 4 = 2560 MB just for the VM's themselves not counting memory being used for the VMware Workstation application and everything else running on the Host including the Host OS itself. With that added to the 2560 MB and the fact that while you have 4 GB RAM installed on the Host nonetheless it is not all accessible on a 32-bit Windows OS. Bottom line is, you have a lot running for the amount of RAM Windows 7 32-bit has access to.
Also make sure that you exclude VMware Workstation and the VM's from being scanned by the Anti-Virus program and this will help a bit in performance.