Just my guess: perhaps a single hydraulic system 1 or 4 operating on the upper or lower rudder could be insufficient to guarantee full deflection of that rudder... hence the increase in VMCG.
This is required only for 1 and 4 because being outboard engines, their failure causes the most yaw; 2 and 3 would require would less rudder.
PS For 600ft-lb: not all 744s have electric demand pumps on 2 and 3, some have ADPs on systems 2 and 3 as well (just like the classic)