The possibilities are various depending on priority setting of the PMGs.
Mode 4 actually gives 5 colours vs only 4 for Mode E, same resolution.
With PRIOR set for multicolour Player mode I believe you can have 23 colour combinations all up, without resorting to DLIs or colour changing tricks.
In Mode E that'd be reduced somewhat. Also the process is by ORing so you'd need to be careful with colour selections to begin with.
Realistically though, you'd probably be better off just using colours you want rather than settling for ones that will work by that method. But still, you'd end up with a reasonable number of useful ones.
The 23 colour combos...
PL0 Ored PL1 / PL2 Ored PL3 =2
PF0 Or combos with PL0/1 = 3
PF1 Or combos with PL0/1 = 3
PF2 Or combos with PL2/3 = 3
PF3 Or combos with PL2/3 = 3 ... total = 23