  We show that Rb_4C_60 transforms from its ambient pressure orientationally
disordered tetragonal structure to an ordered orthorhombic structure
(isostructural to Cs_4C_60) at or above 0.4 GPa. This opens the possibility of
studying the previously observed metal-insulator transition under pressure in
this narrow band system as a function of a continuous variable, rather than
formulating distinct models for conducting and insulating fullerides. No such
transition is observed in K_4C_60 to 2.0 GPa
