Yes, the boot-args method is not working in PB4 — you need to reboot into recovery partition and disable rootless there using the GUI. From the panic logs I got:
*** Panic Report ***
panic(cpu 3 caller 0xffffff7f968ed4ed): "Sandbox unable to determine protected devices"@/BuildRoot/Library/Caches/com.apple.xbs/Sources/Sandbox_executables/Sandbox-442.0.0.1.3/src/kext/iokit_rootless.cpp:415
It appeared to have something to do with rootless, and since i've disabled it I haven't had crashes (although it may also be the fact I upgraded to PB4)...