I think you will be very happy with your decision and I think a good decision, considering your requirements. If 300mm was my max I wouldn't even think about the 3 vs 4 issue (having had the benefit of shooting much heavier lenses on that tripod for a couple of years now).
The important thing is to go through the process of getting comfortable with a decision. That's the tough part . Very difficult to do from a distance.
Also wanted to mention, it's the four-section fraternity, not the four-legged .