Dear Michele,
I strongly agree with Mark and Csilla.
4e9 rays is already a very large number of rays. I have never used a simulation with more that 10e9 rays so far in my life, and that was for extrem photometric simulations.
Truth is you most certainly don't need to model each photon! As with everything in life, physical simulations are a balance between speed and accuracy.
Let's think about tracing 1e16rays for a minute. if 1e9 takes 1 minute [it won't by the way], then you'll need 20years to finish the ray trace...
I would advice you to try with less rays and to simplify your system, always start small, like 10,000 and see what is your noise, and only then start increasing and refining your model until you can live with the amount of noise.
As Mark said, there are probably smarter ways of getting the results you need.
If you are new to non-sequential, I can only suggest you to have a look at our learning plan here.
Best,
Thomas