The Oxford to Heathrow Airline Bus doesn’t go to Heathrow Terminal 4. You get dropped off at Terminal 5 and then need to make your own way to Terminal 4. I found it super confusion and therefore thought I would write some directions to Terminal 4 are to:
1) Go to Level 0
2) Go outside to bus stop 7
3) Take bus 482 or 490.
This takes you directly to Terminal 4 and is free.