I'm adding a subpanel onto my GE TM2020CCU. The subpanel most likely will be a GE TLM1212CCU, for the purpose of feeding four electric baseboards (1500w and three 750 watts), and a water heater.

A GE THQL 2160 60-amp double pole breaker in the main panel

will protect the sub.

My questions:

I plan on using a 4 inch long x 1.25 inch offset nipple between the two boxes. I'll be using #4 THHN to connect the two boxes. Should my ground be #4 BARE or Green? (Can it be #6?

Is this a better way to provide ground to the subpanel (as opposed to grounding through the nipple)?

Note: I plan on using grounding bushings on both ends of the offset nipple, to be connected to the ground bars of each box. Yes, I know that the ground and neutral must be isolated from each other in the subpanel :)