Well is OP trying to cut off 4" or mow at 4"? And if at 4" how is the OP trying to cut at a time. It can simply be deck overload. Here it is hard to cut wet heavy than dry grass. Here I am having to cut grass that is near 6" high plus seed heads. When wet the Hustler Fastrak 54" just clogs up when it is damp but cuts it fine with a little slow down as it still can overload the deck at times. It does needs larger engine than the 20 hp it has but that is what came on it.
Now my Yard Machines 42 has no problem cutting it when dry but it has upgraded engine ( was 16.5 hp, now 21 hp) and heavy duty 5/8 deck belt. I actually mow waist high grass with it at about 1/3 of full travel speed.