I'm reasonably certain it's possible to walk to T5, however it's not via the same ingress that cars use... I'm open to being disproven of course. T4 is definitely accessible, however the T2/T3 central terminal area isn't accessible since the side tunnel that used to be open to pedestrians is closed. As mentioned upthread, take a free bus from the Bath Road if you need to get to this area, or if you're really keen to walk it then walk to T4 and take the free transfer train from there (this is landside–don't try to get airside at T4 if that's not where you're departing).