This summer I stayer at the Sheraton Skyline at LHR and explored the neighborhood a bit. It looked to me like one could at least walk to T5 by going left out of the hotel and taking the short perpendicular street toward the airport runway fence at the first light, then going left again at the T. I was watching takeoffs and landings and there seemed to be a sidewalk as far as I could see. T4 looked similarly possible but I'm less sure of that.