I carried both a 4" and 3" GP100 at times in the past. For holsters of the OWB variety I had great success with both the Fobus paddle holster and the Bianchi AccuMold holster. For the environment you will be in I would recommend the Bianchi as it has a thumb strap and is made of tough ballistic type nylon. These are very ruggedly made holsters and will hold that six-gun safely in place, plus it offers excellent coverage to protect it from the elements as well. I looked on Midway and they have some in stock for around the $50 mark. I have listed the product number and description below so you can see an example of the AccuMold I am speaking of.
Product #: 613242
Bianchi 7105 AccuMold Cruiser Holster Right Hand S&W K, L-Frame 4" Barrel Nylon Black