All the tests were done with firmware revision v2.07 and now we are up to v2.16 so there could be quite a few changes made to make more Media Compatible !!!
I have no problems with Ritek 4x both -R AND +R DVD'S they work just fine and so far no Coasters !!!
The ones I would suggest you avoid are the Mirror 4x -R's as I have loads of Coasters for my Coffee cup in the Office using these !!!
I have recently tried some Arita 4X DVD+R I purchased from :- http://www.blankdiscshop.co.uk
and so far I have no problems at all !!!