Transfers at T4 have been subject to security checks since within a week or two of its opening on April 12th 1986. The requirement for reconciliation of transfer baggage came in at the same time,- well before Lockerbie,- and this created enormous headaches for BA although they implimented the checks immediately. The building and systems were designed without either of these things being a requirement, so the facilities have never been ideal.
The range of points of origin of transfer passengers at LHR and their sheer volume, and the fact that people from all of them can mix freely, means that it is impossible for some to be subject to transfer security checks and others exempted.
Undoubtedly the current searches will therefore continue at T5 but hopefully as the building has been designed with the full knowledge of what is now required they will be conducted more easily and in better surroundings than before