VAFFPAX
There are local buses that link T4 and T5 for free and they are fairly regular. I have also seen another bus, not the BA staff buses but some other sort of BA charter bus that had signage along the lines of T4-T5. It was going too fast to get a decent look, sorry.
Heathrow Express is too much hassle for me, personally. By the time you've made it all the way downstairs, waited for the train to come in, the security check, then boarding, you may as well have just caught the bus.
Hopefully, once the moves are completed it won't be too much of an issue.