this is an example of turning on selective G4 verbosity
this particular configuration will set G4 tracking verbosity
for every 2nd event (EventStep=2), for events from 0 to 6; 
G4 tracking info printed for each 100th track (TrackStep=100),
for track numbers from 0 to 10000; if you wish, for example,
to print tracking info for every 10th track, set TrackStep=10,
or for each track - TrackStep=1
NOTE: don't forget to turn on VebosityLevel - otherwise only
info on the particle ID and track ID will be printed but nothing
on position, volume, etc.

For example usage, see also 
SimG4Core/Application/test/runP-WithPionAndQGSP.cfg

TO USE:

in _cfg.py file:

process.g4SimHits.Watchers = cms.VPSet(cms.PSet(
    CheckForHighEtPhotons = cms.untracked.bool(False),
    G4Verbose    = cms.untracked.bool(False),
    EventMin     = cms.untracked.int32(0),
    EventMax     = cms.untracked.int32(6),
    EventStep    = cms.untracked.int32(2),
    TrackMin     = cms.untracked.int32(0),
    TrackMax     = cms.untracked.int32(10000),
    TrackStep    = cms.untracked.int32(100),
    VerboseLevel = cms.untracked.int32(1),
    DEBUG        = cms.untracked.bool(False),
    type         = cms.string('TrackingVerboseAction')
))