I've walked T4 to Park n Fly lot a few times - where LAXit now sits on part of the lot.
I've used a couple different routes - Along the upper or lower level past TBIT then past T3-T1; or the other way to T5/6then across to the T1 side and over to the lot. Time-wise it doesn't seem to make a lot of difference - about 15 minutes from the gate in T4 to the lot.