"Nightmare" is a strong word but this is FlyerTalk; a "bit of hassle" would perhaps be more accurate. It is just that there is not as good a transfer landside as there is with other terminals. From 4 to 5 it's not so bad, using the tube, but not in the reverse direction. The Heathrow Express does not go to Terminal 4 so you need to change at T1+T3. Long passageways and 15 minute service frequencies don't help. Airside is a lot faster.