So to be really clear

Arrive at T5. Follow purple signs for "flight connections". Take the bus for T4. On arrival there follow signs for immigration and clear. Once through the duty free shop veer to the left and take the escalator opposite WH Smith

Stay at YOTEL

Go back down the escalator and through the exit next to WH Smith and go all the way to the left. There are two bus stops, one of them is marked "towards Heathrow Terminal 5". Then take either the 482 or 490 bus with destination "Heathrow Terminal 5" on the front - it's free, just get on the front door, you don't touch in like you would normally in London. This will drop you at T5 where you can take the lifts up to departures and through security.