The internal BEC is only rated for 3s Lipo :). It sounds like it's the BEC chip that's shutting down, which would be due to the extreme efficiency of it when running ~19.8v.
SH Z-Car, Custom Crawler, 8s Savage, 12s XTM XLB 1/7 buggy, 4wd 4-link rear/IFS Pro4 truck, Custom Hyper 10 Short Course, Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz