Edited by ryanjsoo - 1/21/13 at 8:39pm
The Olives fit on the W4s, but the SE535s still isolate more. This is because the nozzles on the Westones are rather short, and don't go in as deep into the ears as with the Shures. I'd take the sound of the W4s any day over the Shures.
There's isolation by design, and then there's isolation by fit.
The first one cannot be improved, the second one can.
You can use Comply tips to see if they work well. They fit better than olives because they are foam based.
As far as I can remember during my testing with both of them, the Shures did better than Westones in isolation, but Westones were not bad at all, only that the Shures were better. Especially with the included P-Series, I think the Westones are good enough for you. If you want some real crazy isolation, go for an Ety.