Recently I had a trip from BLQ-LHR-ABZ which required self transfer from T3 to T5 we were HBO - still took about 90 minutes to get from T3 to T5 (we just missed the HEX to T5) immigration was laughably long, this on top of the flight being 90 minutes late just added to the fun. Four hours might be enough if your plane is on time……I’ve no idea how long the T4 immigration queues are - it’s been a while since I’ve been there.