I had Kreiga bags on my Gen3, strapped down as tightly to the pillion seat straps as I could, but they always fought me for the rider's seat when braking.
I had mixed success with SW Motech Blaze panniers - first trip they worked perfectly. Second trip, the exhaust side pannier melted. The Gen3 end can (OEM) was a fair bit higher/more upswept than the previous models, and I suspect that the panniers/rails weren't sufficiently redesigned to cope. End can on the Gen4 seems lower, so should be fine.
The most reliable solution I found with the Gen3 was to fit a Renntec sports rack and use that to anchor a 70litre Oxford duffel bag to the pillion seat. Obviously, the rack looked shit. ISTR Alex having a similar (but less shit looking) solution of a Hepco & Becker SportRack replacing the pillion seat - aluminium plate with numerous holes and slots to securely attach soft luggage. Luis-moto have one listed for the Gen4 - to order, but I can't find anything on Hepco & Becker's own website. When they eventually materialise I'll probably get one and throw some rok straps on it.
As I have an SW Motech Evo tank bag (daypack) for my winter hack (R1200R), I have installed an Evo tank ring on my Gen4 - tank bag gets in the way slightly, but on the road I find that the utility of having it more than makes up for a little bit of getting in the way. Had the Wunderlich tank bag (crescent shape) on my Gen3. Never got in the way, but did scuff the panel. Dunno if/when they are going to do one for the Gen4.