Rickresource Rickenbacker Forum

Those are 3/8ths nuts I believe, a nutdriver won't fit unless it's very short, you can use a regular old box wrench in there just don't torque it up too much all at once and make sure you take the string tension off before you turn it although you are loosening instead of tightening them, I don't mean loosen the strings just push the neck back somehow to relieve the forward pull of the strings.
I use nutdrivers for all my others except for the early 4003 I have.

My old Craftsman 1/8" red/clear-handled nut driver works great - it is hollow a ways beyond the nut part so it fits nicely.