At a surface speed of say 650 ft/min the rpm at a diameter of 4.5" would be close to 552. Accordingly any clamping speed above that rpm would result in the commanded surface speed being maintained through the whole dimension range form 6.0" to 4.5".
To calculate RPM given the surface speed:
RPM = CS x 12 /Pi/D
CS = Cutting speed in ft/min
Pi = mathematical constant approximately = 22/7
D = work or cutting tool diameter in inches
To calculate surface speed in ft/min given RPM and work or cutting tool diameter.
CS = (RPM x D x Pi)/12
The same references apply as for calculating RPM.
Your code could be
G28 U0. W0.
G96 S478 M03
G1 X4.5 F.004