4-6 hours is a very short trip. A tip is very subjective and here is how I look at it.
If a guide picks you up and really isn't there for you, doesn't really show he is willing to put out then $0-$20. A $20 tip often says more than $0 if the guide knows you know the gig.
An average guide will get 10% on the low side to 20% on average. A guide that exceeds your expectations in his ability to teach, entertain, find fish and get you into fish will get 20-50%.
I am a guide and I charge $425 per day and $350 for a half day. My average tip runs $80-$100. My best tip has been from a couple who didn't know how to fish. Came on the boat extremely stressed from work. The biggest fish the wife caught was an 10" trout. But by the end of the day they were smiling ear to ear. His comment to me was " Wow, I don't want to get off the boat. I have never had a day change my attitude so profoundly." He tipped me 100%.
On the same note, someone purchased a trip I donated for a single dad of three kids, working and going to school full time. I busted ass for them cause I knew the dad didn't get much time to play. The kids were good kids and we had a great time. The dad gave me a $20 tip and it was more than I ever expected. He couldn't afford that but if I gave it back I probably would have insulted him. I thanked him for the great time.
I don't work for tips. I get them but it is my job to be everything I can for each and every client. The tip is purely the icing on the cake.