You can walk all the way from Terminal 7 to TBIT... You basically walk to the "root" of T7, there you can turn left before leaving the secure area. This takes you to T6, which you enter and halfway down T6 you can enter the tunnel, that will take you to T5 and T4. In T4 you walk again to the "root" and should find the signs to get to TBIT. If I remember correctly, the signs will already start at T7.