1. I haven't been to T4 in a while, but you will not be able to enter the departures area after arriving. (It's like International arrivals in the US in that regard.)
2. Unsure-- like the US, the baggage claim is between Immigration and Customs, so it's not like someone could wander in off the street and pick it up.
3. You can take the Heathrow Connection, runs every 10-20 minutes and leaves from a station in the basement of T4 and runs to a station that serves T1-3. From there, it's a 5 minute walk (and an insanely long elevator ride) to T3.
4. Talk your way into the VS arrivals lounge at T3 :-). Or maybe head over to T5 and check it out. I don't think there's much before security at any terminal at LHR. Are you going into Central London? The Heathrow Connection will take you there in 35+ minutes, and isn't that expensive (maybe 10 pounds??). No need to wait around the airport for three hours for your co-workers to show up just to share a cab.