First question is, what does the 20Amps derated mean to 30amps PDU's?
I am looking to see if I can run 4 rigs on a 30amp PDU from my Dryer connection. I live in an apartment and I have no other of changing anything, so I'm sacrificing my washer and dryer, I can do laundromats.
It depends on the PDU - a few are "continuous rated" in which case they will probably be listed as "24 amp", most are not in which case a 30 amp unit should be derated to that 24 amps.
The drier circuit is ITSELF probably a 30 amp circuit, in which case you would have to derate to 24 amps continuous usage ANYWAY.
If your rigs eat 1320 watts "at the wall" or less, you should be fine with 4 of them.
If your voltage at the outlet is higher than 220 volts, you can go proportionally higher on the wattage per rig safely.