I've had the bus take 45 minutes; I don't use it these days either.

I walk - making sure when I exit T4 I do so at the departures level roadway to save a couple of minutes.

Security in the evenings at TBIT generally takes no more than a few minutes these days.

If you really wish to take the bus, it's scheduled to depart every 30 minutes from 4pm; on the hour and half hour. It normally pretty timely - perhaps occasionally departing a few minutes early.