I assume this is a domestic flight? 4.5 hours sounds about right. Budget 2 hours (minimum!) for driving, at least 45 minutes for rental car return, and up to 30 minutes for TSA. That leaves you with about 75 minutes before departure time if everything goes smoothly. Traffic on Thursday afternoon on CA-14/I-5/I-405 from Palmdale to LAX will be murder... And there isn't really a better route to go.