Yes, four hours is about right. I did this transfer a week ago Sunday morning, a time I thought should be relatively quiet. The queue for the bus to go from T3 to T1 went from a packed waiting room, up the stairs, and around the corner. Nice and stuffy in the waiting room, some people looked ready to pass out. That pleasant experience took almost an hour. Then to T1, where we had to wait in a queue of buses unloading. Then into another waiting area, where a guy was gating traffic up the escalator for crowd control. Up the escalator and line up to get into the FCC, with 3 lines merging into one. The allowable bag size has increased, so that's not much of an issue anymore. But they have agents looking to make sure there's only 1 bag/pax, and there's lots of amenity kit contents being tossed. Then into the madness they call the FCC. The FastTrack there is for BA J or F only. Fortunately, I was able to use this and therefore only spent 45 minutes in line. Finally got through all this to find that my connecting flight was closed, even though I arrived early and had 2.5 hrs to transfer. A nice agent was able to get me on the flight, but there were many, many others who were rebooking their missed connection.