(angles between prisms are approximate and not necessarily to scale)
2-Prism lens:
(view of screen)
A: Primary laser beam.
B: Reflection from side of prism A
C: Reflection from side of prism B
Both reflections (B and C) can be removed by masking the sides of each prism (the sides marked with A and B in the upper diagram). So reflections on the screen can be removed from a 2-prism lens system easily.
4-Prism lens:
(view of screen)
A: Primary laser beam
B: 2 Reflections from side of prism A
C: Reflections from side of prism B
D: 2 Reflections (unknown source)
Again, reflections B and C can be removed by masking the sides of each prism (the sides marked with A and B in the upper diagram). However, reflection D is not removed by any masking. I haven't found the source of this reflection yet, but it seems to be caused by the dual prism arrangement.