If you really only have 1 hr 25min between published arrival (brakes on) at T5 and published departure ( brakes off) at T4 then good luck....It"s probably "do able" if there are no delays
at all to the inbound service, no delays in T5 ( which is generally now working OK) and the buses between the Terminal's run on time
I reckon it's less than a 50% chance that you manage it - dare I ask why you've ended up with such a tight connection?
( edited to add: Yellow Sun - our posts' crossed,
For interests sake where did you find the minimum transfer times between LHR terminals - I failed when I looked at ba.com)