Other than the obvious is there much difference between the 4 vs 6 position collapsible stocks?
Is the buttplate shaped different or is one more ergonomic?
I believe the 4 pos is a little shorter (3/4")
I think the 4 pos has an extra sling mount?
I have a 6-pos RRA and would recommend it without any hesitation.
Look in EE. There is a deal for $72 and it comes with a wrench and free shipping from RKPrecision