For convenience I'm installing a 100 amp subpanel within one foot of the main panel, which is 200 amps.

NO new circuits are being added; only moved from main panel to subpanel.

Three THHN #3 wires and a #8 EGC wire will be run through 1 1/4 inch PVC.
(Other conduits for circuits)

In the subpanel will be 6 20 amp CBs and two double-width 240 volt CBs.

The 240 volt CBs comprise about 25 percent of the load.
No neutral current on these two circuits.

So, can I reduce the neutral wire size to #4 ?

Thank you.