Personally haven't had the chance to shoot GH4 either. Only GH3 so far. I might get to shoot both in Jan/Feb, will see if I can do a comparison then. On paper though, the differences that standout most (to me) with the E-M1 is that it has limited codec/framerate options (only 30p) and has no headphone output jack.
Anyway, let's not OT too much in this 40-150 thread. bhtan83, if you're interested, I can pull these posts out and create a separate thread for discussion, I'm sure there'll be current GH4/E-M1 owners who can chime in.