Hi
GK4 was stopped to be developed and GK5 was presented in it's place, so it's always better to use GK5.
Do You have multiple languages used on Your site ? Maybe You need to specify from which language table categories should be picked up. Or VM is not configured properly in languages section.
Also not all module styles support Virtuemart. Try with Normal mode.
One more thought. Since GK4 and GK5 are basically same module, using both at the same time may cause some problems since GK4 is loading old libraries.
If You need help with that please send me an access to Your joomla panel and an url to Your site.