Tallygear has just released a silicone skin case for the G4. This is what I have wanted all along and hopefully it won't disappoint. Currently red is the only color available, but orange, purple, blue, and black can be pre-ordered with delivery expected soon. You can check out the multiple cases available at www.tallygear.com. In addition to the new silicone skin, there is a lycra case available in a rainbow of colors and patterns and also a more protective neoprene case.
I also just found out that you can get custom clear screen protectors for the G4 at http://pumppeelz.com/collections/dexcom-g4-peelz.
You can get decorative decals at both sites.
If you'd like to read more, I published a blog post on Monday about Dexcom cases: http://testguessandgo.com/2014/04/21/dexcom-cases/