With a 10-hour layover you may wish to consider a day-trip into London to stretch your legs and have a nice meal. You don't say what travel class or airline, but there are arrivals lounges in both T3 (AA) and T5 (BA), though these are not places you want to be for more than a few hours, and which close in the early afternoon anyway. With OWS you can access the BA Galleries Club lounges (three of them) in T5, which are better than than any other option you have. Again, 10 hours is a long time. Don't bother with trying to get into T4; it'll be a pain to get into and out of, and may not even be possible to do airside as you don't have a BP for a departing flight from there) and there are no OW lounges for you to access there anyway.