If you are really tapped in third, then you're bike should be able to pull fourth with only a minimal clutching. If you can't pull fourth, then either you arn't really tapped in third, or, like Kay Dee Exer said, your bike is geared too high. Sometimes, you think you have it pinned when you don't. Try twisting the throttle all the way to the stop in third as you hit the receiver, then hold it there all the way through the trough to the take-off of the next jump. If you are already doing this, and you still need more speed, then try clicking fourth in the air. Clutch down the receiver of the lead-up jump, but get all your clutching done before hitting the take-off. Like Gomer said, you probably don't want to be clutching up the face of the take-off. If fourth still won't get you over, then practice pre-loading to get a little more boost out of third gear.