I think the difference is really just the replaceable pad feature. The arm material I'm pretty sure is the same though the 6's are advertised as lighter weight. I actually think the 4's had a nicer finish on them (cosmeticly speaking).

The 6's stop very well, but the front took a while to set up perfectly to avoid horrid screaching that would send people running for the hills

I haven't had any problems with the 6's other than the initial squealing while I had plenty issues with the 4's.. they seemed to need constant adjustments.. first time adjusting the tension screw and it stripped.. I've adjusted the 6's several times to tune them in and have had no problems.