I found this case yesterday in the trunk of my car. I believe I bought it years ago to jog with my Blackberry Pearl. It has a detachable belt clip that swivels and is almost exactly the right size and design for the G4. It even comes with an armband and corresponding slot which could be used with a belt - although that would orient it vertically.
The tag on the inside has a model number of CRC90677 which I was able to find on amazon for $10.