There's a similar thread here.

http://www.flyertalk.com/forum/u-k-i...gw-lhr-t4.html

Probably the coach would be cheapest and easiest if you have luggage as you dont need to change but depends on the time of day for your 3 and a half hour transfer.

Where are you coming from, which terminal, wil you have baggage and what times of day? This will affect how long it takes you to get through LGW and whether the coach is likely to take longer due to traffic. Also which LHR terminal are you going to?