* 4x RTX 2060s at $350 each ($1400). will fold at ~4,1 M PPD,
running at 130W each (that's about 625W at the wall).
* 3x RTX 2080s at $700 each ($2100) will fold at ~4,2 M PPD,
running at 150W each (that's about 525W at the wall).
* 2x RTX 2080 tis, at $1200 each ($2400), will fold at ~4,4M PPD,
running at 230W each (that's about 575W at the wall).
The wall power draw number, roughly equals annual electricity cost, so it seems that:
- the 2080s are most economic for folding in the long run,
- the 2060s are cheapest for folding, with a max of 7 years, 24/7 folding.
- and the 2080 TIs are the fastest (duh)

Same Intel quad core benchmark system is used, under Linux.
As long as you're using Linux, 4 CPU cores work fine with 4 dedicated GPUs.
No extra CPU core is needed for Linux.
One could use a lower powered quad core (45W TDP) 2Ghz CPU for the 2060, and save ~20-25W on electricity; where as the 2080 TI needs a 3Ghz (~65W TDP) CPU, which brings total system overhead to ~100W.