Hi wark286,
Yes 4K mtu is supported in IPoIB for vSphere 5 as explained in the User Manual.
You basically need to configure the SM to enable 4K mtu for IPoIB (check your SM user manual, the configuration in the SM leve may vary depending on the SM provider, if you're using opensm running on a host, then see page 20 in the driver User Manual), then configure the vSwitch and the VM to use 4052 bytes mtu, check vSphere doucmentaion for more details, if you like to do that through the vSphere Client GUI, this guide can take you through the required steps.
Note, while in Ethernet Jubmo Frame are up to 9000 bytes, IPoIB Jumbo Frames are 4052 bytes.