When using a 20MHz external the first config line will look like this

@ __CONFIG _CONFIG1H, _HS_OSC_1H & _FSCM_OFF_1H & _RC_OSC_1H
And then this
DEFINE OSC 20

"HSPLL" would try to make the running speed 80MHz... I have my doubts if it would work. I have heard some folks tell of these running at 64MHz (and they did not use water...) but the data sheet claims 40MHz is the limit....

It is correct to assume with HSPLL the multiplier is always X4?
With this chip, yes.

From the data sheet
2. XT Crystal/Resonator
3. HS High-Speed Crystal/Resonator
Table 2-1 and table 2-2 are also worth looking at. 2-1 is for resonators without built in capacitors and 2-2 is for crystals. If resonators that have built in capacitors are used, even if they are 4MHz use "HS".