Under BESS4, can I install a 20kWh BESS and claim PRCs, then 12 months later install another 20kWh BESS and claim PRCs for the second system?

No, BESS installations can only be claimed once per site.

The PDRS Rule does not prohibit an existing battery at the NMI for activities BESS4 or BESS5, however, a previous implementation of either BESS4 or BESS5 must not have been conducted at the site.