I was looking on the BA website and it said that it would take an estimated 90 min from terminal 4 to departing from terminal 3 at Heathrow. This will be my first connection through Heathrow. I have 1 hr 40 min between flights which I had thought would be fine, but now I'm not so sure. Is this just the airline's liberal estimation? Any info would be appreciated. Thanks in advanced.
Transfering from terminal 4 to any of the other terminals is a nightmare and it will probobly take about 90 minutes. The connection from terminal 4 is via bus, and sometimes during peak periods, you will have to wait on line for that bus. Once you arrive at terminal 3 you will have to go through a complete security screening again. If your connecting flight goes to somewhere in the UK, you will have to go through a immigration check. If the flight is outside the UK then there is no immigration check. To speed things up make sure your carry-on baggage is compliant with both the TSA and European regulations.
It's often not that bad. I made it from Terminal 4 to Terminal 1 in about 20 minutes - only to find out that my connecting flight was delayed for an hour.
You should be fine, but if your incoming flight is delayed, don't hesitate to make the staff aware that you have a tight connection - I was permitted to go ahead of the line at security and at immigrations (well not really a line there).
Also, to ensure a quick trip through security make sure you have just ONE bag and that it's within the allowed size (www.baa.co.uk). UK regs are different for now from the rest of the UK, and you could be delayed at security if you have more than one bag.