I am afraid that I am unfamiliar with GEANT4 (other than knowing of its existence). I did find the GEANT4 webpage and a number of other related pages (there is quite a steep learning curve with such a powerful program!).
https://geant4.web.cern.ch/
I did learn and use MCNP (a powerful monte-carlo radiation transport program from the Los Alamos National Lab). Alas, the US Dept of Energy has made this program somewhat expensive and difficult to obtain unless one is affiliated with particular US nuclear programs or universities. There are a number of free resources that one can access to learn about monte-carlo methods. A very good text book is Reactor Shielding for Nuclear Engineers by Schaeffer, 1973. Downloadable from the link -
https://www.osti.gov/biblio/4479460/
https://www.osti.gov/servlets/purl/4479460/
There is an MCNP website, which has some useful, freely downloadable documents -
https://mcnp.lanl.gov/reference_collection.html
One document in particular is a compilation of material compositions including that of many scintillation detectors -
[broken link removed - Steven]