Dead easy from Heathrow - you can get the Heathrow Express into Paddington and it's a 5 minutes taxi ride
Maida Vale would be a nice place to stay actually, nice area - Regents Park is fairly close which is nice to wander around, at a push you could walk down to Marble Arch and then around Hyde Park or the West End.
Central London is really not that big, with 4-5 days I would recommend just walking around everywhere, if you want to see the City etc, start off there and walk back West towards your hotel. You'll see loads more than you would if you get the tube round everywhere, much more pleasant as well.