When the diffraction effect affects your image it manifests itself in a loss of contrast, loss of edge definition, and an overall look of softness. The effect is going to hit regardless of effective MP count, or whether you are shooting stills or motion picture. I would expect it to get worse on the GH4 due to the higher resolution and rendition of detail (I'll have to try it, just got mine).
I had it hit real bad on a 6.3MP Canon 300D (the original digital rebel) when I forgot to check the ISO, night before I had been using ISO 1600, the highest on that camera. The next morning I began shooting period re-enactors at Ft Concho and was wondering why I was having to use the smallest aperture on the lens (kit 18-55 f3.5 - f5.6) with the highest shutter speeds. Minimum aperture was marked f22 but at full tele was likely really effective f45 or so.
Fortunately I caught it fairly early and dropped ISO down to 100 but the first dozen shots were almost useless. Worst case of diffraction I've ever seen!
I would avoid variable ND filters or "faders". They have 2 panes of glass and in addition to causing extra internal reflections have also been sometimes causing loss of definition at the telephoto end of zooms. I carry Tiffen ND 1.2's (4 stops) in my kits and need to acquire a couple of ND 1.5's (5 stops).