Hi,
since version 11.0 (may be earlier) you can start explicitly the SERIAL mode in a MT compiled geant4.
env G4RUN_MANAGER_TYPE=Serial ./exampleB1 run.mac
But be aware SERIAL mode and MT mode with /run/numberOfThreads 1 are not excatly the same. See here: https://geant4-forum.web.cern.ch/t/user-s-hook-for-state-change-doesnt-work-in-mt-mode/7519
Henning