So I did this on Saturday and it was great. Luckily had fantastic weather and avoided the thunder storms later in the weekend. By the time I'd transferred from LGW and showered and changed after an overnight from LAS it was 13:30 when I got out of T4. I think making Paddington was a bit ambitious. I got as far as Greenford. Paddington would have been another 2.5 hours. I walked until 17:30 and covered 12 miles then stopped for a pint and some fish and chips and got the tube back to T4 for a 20:30 flight. The walk from T4 to the Grand Union Canal basically breaks down into 4 stages:
1. T4 east to River Crane along a mix of perimeter road and A30 - not particularly interesting but what I expected really. I was able to walk right out of T4 down the exit ramp from the arrivals drop off to the A30. As someone mentioned you have to switch back to the perimeter road after Hatton Cross to get to the River Crane. Earhart Way is blocked to traffic and looks closed but when you get up close there is still pedestrian access.
2. A30 north to A4 North along River Crane - this was the only disappointment - the footpath along both banks of the river was completely waterlogged. You would need wellies or waders to cross it. I ended up cutting through the Cranford housing estate on the East of the River.
3. A4 north to M4 through Avenue park and Cranford Park - this is by far the best part of the walk. Avenue Park is a nice green open space and Cranford park is a lovely oasis right next to Heathrow. Lot's of open parkland.
4. M4 north to Grand Union Canal - Not as nice as Cranford park but mostly through woods so a different sort of scenery. Once you cross North Hyde Road you have to walk through an industrial estate to get to the Grand Union Canal. From there it's a short walk to Bulls Bridge and then you can either go straight on towards Brentford or turn left and head towards Paddington.
I would definitely do it again if I had a long stop over and it was a nice day. Next time maybe 'll start at Paddington and go from there. GPS track and some pics attached.