  ```
from dd4hep import DD4hepSimulation
from dd4hep.detector import Detector
from dd4hep.geometry import Geometry
from dd4hep.material import Material
from dd4hep.particle import Particle
from dd4hep.simulation import Simulation
from dd4hep.tracking import Tracking
from dd4hep.userinterface import UserInterface
from dd4hep.utils import get_code

# Initialize the kernel and load the geometry from an XML file
kernel = DD4hepSimulation()
kernel.load_geometry(get_code("DD4hepExamplesINSTALL/geometry.xml"))

# Import constants from the kernel's detector description
constants = kernel.detector.constants

# Set up a Geant4 instance with a tracker
geant4 = Geant4(kernel)
geant4.set_tracker(Tracking(kernel))

# Configure the user interface
user_interface = UserInterface(geant4)
user_interface.set_tracking_field(constants.tracking_field)
user_interface.set_event_actions(constants.event_actions)

# Set up a particle gun with a gamma particle, energy of 5 keV, and multiplicity of 1
particle_gun = user_interface.add_particle_gun()
particle_gun.set_particle(Particle.gamma())
particle_gun.set_energy(5e3)
particle_gun.set_multiplicity(1)

# Set up a tracker and a physics list, adding various particle groups and processes
tracker = geant4.get_tracker()
physics_list = geant4.get_physics_list()

# Add particle groups and processes to the physics list
physics_list.add_particle_group(constants.particle_groups.electron)
physics_list.add_particle_group(constants.particle_groups.positron)
physics_list.add_particle_group(constants.particle_groups.gamma)
physics_list.add_process(constants.processes.electromagnetic)
physics_list.add_process(constants.processes.charged_hadron)
physics_list.add_process(constants.processes.photon_hadron)

# Execute the Geant4 instance
geant4.execute()
```