My impressions from the previews and photo comparisons floating around is the G4's screen isn't quite as good as the S6 and the camera in auto isn't quite as good as the S6 although those differences are minor. Full manual controls and DNG output tilts the G4 camera heavily towards enthusiasts. The S6 wins on design, fingerprint scanner, quick charging, and built-in dual wireless charging, although these aren't important to a lot of people. The G4 (presumably) wins battery, but we shall see. Note that 5.1.1 appears to be pushing out already, so the S6 may have it across the board by the time the G4 is released.
Personally, the G4 is ruled out because of size -- 5.5" is too unwieldy for one-handed use, but the larger screen may be a benefit to some.