You definatly want to run 4 wires plus install a ground rod. The #6 ground wire is important cause the ground kit on the panel won't accept a #4 wire, but, you can purchase a connector to accomidate the #4. It connects to the ground bar. What is the make/model of panel you're installing?
And there are also important rules when installing a subpanel, like making sure you throw away the green bonding screw so the ground and the neutral buss bars are separted.
Here's a link that'll show you what is required based on your particular situation. Scroll down on the left side of the link for diagrams as well.
http://www.selfhelpandmore.com/homew...rage/index.htm