After being a bit chicken and doing 4 x W3, I have just got back from my first W4 run! The thought of it was worse than the actual experience. Not only did I find the 3-minute runs somehow more "do-able" than they were last week, when they were the pinnacle of my running activity, but I actually found myself getting into a rhythm for my 5-minute runs. (Running along our local canal towpath on a glorious sunny afternoon helped as well!)
I took note of all the great advice on this forum. The things that seemed to help me the most were: keep a nice, easy pace; and don't worry about going really slowly - as long as you don't stop running.
The extra thing that I found helpful was to make a conscious effort to keep my back, neck and head straight - when I get tired I tend to crouch forward, but keeping my body aligned makes it all less effort.
I hope this isn't just beginner's luck - on to R2!