Can I turn off IPv4 and just use IPv6 on my Mac?
I assume you're talking about on your Mac since you don't specify.  Yes you can do this but in all likelihood, you won't see much other than your internet connection probably won't work.  Most ISPs (Internet Service Providers) are not yet ready to handle IPv6 traffic and many won't be right up to IPv6 day on June 8th 2011.  Many won't be ready beyond then either.  But it won't do any harm to try it out if that's what you're worried about.  As an aside, the last batches of IPv4 addresses were distributed recently.

To turn IPv4 off go to System Preferences > Network > Advanced > TCPIP  

Then on the drop-down list next to 'Configure IPv4' select the 'Off' Option.  Then beneath that, you can set the 'Configure IPv6' drop down menu to whatever you want.