To jog the 4th axis ......... Get into JOG mode in VR Milling and use the . and / buttons on the keyboard
To align the 4th axis to the 0 Degrees arrow..... Using the . and / keys jog the 4th axis until the 0 on the graduated scale is in align with the 0 Degrees arrow. Disconnect from the machine and then reconnect VR Milling and 0 degrees on the software will coinside with 0 degrees on the graduated scale.
N.B. If the axis is rotated say 30 degees and you want to get it back to 0 degrees, just go into MDI mode and type A0