The G4 is actually not that common now, as it was only produced for three years before being superseded by the LM.
The surest way to tell is to unscrew the main jet and pull it out. If the jet has an extended 'nose' beyond the thread, it's an LM.
If it's a G4, it won't have that, and the 'spray tube' should come out with it.
See also this past thread for more detail:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=38908
