The last departure from T4 looks to be at 10:50PM, so the T4 transfer bus wont be running when the OP lands anyway.
Also as already mentioned, these are airside so if the OP has checked luggage they are of no use.
I am also not convinced they are available for arriving passengers clearing customs at LHR. Whenever I have taken one of the buses between terminals airside, I think they have asked to see onward flight details.
Really the Heathrow Express / Heathrow Connect option is a lot better than buses if it is still running. Last T3 - T4 train seems to be about 23:35 each night.