I work in a passenger assistance role in the Heathrow terminals and we advise passengers to use the Heathrow Express shuttles to transfer between terminals 2/3 and T4 or T5, rather than use the Piccadily line, because of potential Oyster or CC payment problems. There are 2 travelators between the tube station and terminal 3 or Hex, and one travelator to T2. If transferring between 4 or 5, it is easier to use the 482 or 490 buses, which are free. Connecting passengers, unless they have good reason should not become landside, and should use the airside buses.
The Hex inter terminal shuttle runs every 15 minutes; Heathrow Connect services normally terminate at T5. As I use it for my travel, I think I was the only person on it from T5 the last time I used it.