You might save a little bit of time; most likely, you would not The route isn't that much shorter. If you look at a map of LAX, the tunnel connects T4, T5, and T6 near their midpoints. So, basically, you're saving a walk halfway down terminal 6 and halfway up terminal 4. (You can see the tunnels in the United app -- pull up the LAX map and look at the Baggage Claim level). Even if there's no line for Clear / Pre, you still have to stop, show ID, get scanned, wait for the belt, etc.