I can consistently play high 3 star maps (3.8-4) of any length, but as soon as I attempt maps in the 4* to 4-2* range, I can no longer full combo and my accuracy is way worse. How do I overcome this?
Maps at that star rating start introducing more complex note density and patterns, so you'll want to just keep playing and getting used to them. But you'll get there!
4* maps introduce new types of patterns. You can brute force learning them with playtime or you can play half time to learn the patterns manually. (Editor works too.)