The PLL multiplyer will multiply x4 whatever your OSCILLATOR is running at. There are no other options... it's either x4 or OFF.

You need a 5MHz Crystal or Resonator if you want 20MHz using the PLL Multiplier.

The Internal Oscillator does not have 5MHz as one of it's selectable options (neither does it have 20MHz).

So if you WANT 20MHz, you'll need a 20MHz Xtal or Resonator, use HS Mode and have the PLL multiplier OFF (or 5MHz Xtal or Resonator, HS Mode and have the PLL multiplyer ON).

Section 2, Datasheet DS39631A explains it in great detail (for the 18F4520 chip) which you said you were using at the start of this thread.