There are escalators between levels 0 and -1. From Level 0, you go down the first set, then turn right and go down the second set, to platform level. The left-hand escalator is normally the one you need. See the Environs page for details.
In practice, it may be easier to use the lifts, particularly from the higher levels.
HEATHROW TERMINAL 4 Station
© 2012, Terry Robinson