Beam splitting experiment with single photon

[entropy15]

Consider the above experimental setup.
Here the source can emit one photon at a time.
The output from the source is sent to a beam splitter which divides it into three possible paths of equal probability.
Splitter 2 further splits the path 3 into two paths of equal probability.
We have photon detectors M1, M2, M3 and M4 mounted at the end of the paths as shown.
Considering this, what is the probability that the detector M3 or M4 clicks?

Would it be 1/6 or 1/4?

 

[naima]

The M1 and M2 probabilities are not modified by what happens after BS2 (the setup has a tree structure) so 1/3 1/3 1/6 1/6