Last weekend Anna and I finished cycling (in stages) from London to Fishguard on National Cycle Route 4.
The two bits we'd not covered were Swansea-Carmarthen, and Pontypridd to Newport. Very different from each other: Swansea-Carmarthen was almost entirely flat and traffic-free until the final miles, with 21km of glorious, wide 'Millennium Coastal Park', a peaceful railway path, and some judicious connections including a landmark new bridge.
Pontypridd-Newport was a more typical NCN mix. A railway path was followed by a bit of ducking and diving around housing estates to end up at Caerphilly Castle; a bit more housing estate led to some lovely new winding riverside paths, a railway line on the side of a valley (excellent views), and finally a typical NCN selection of country lanes. Oh, and a really annoying gap in the route at Newport, which we didn't know about until having tried (and failed) for about 1hr30 to find where the route had gone.
So NCN 4 is now pretty much complete on OSM - the first long-distance route to be so. Newport is obviously a lacuna; there's a 100-metre or so gap in Carmarthen, too, where the route seems a little imperfect awaiting a new section for Connect2; and there's a short gap in Pontypridd, too. On bits that others have mapped, a couple of streets are missing near Greenwich, and there's a tiny break in North Bristol. But none of these are more than very short gaps. More useful is that several 'braids' of NCN 4 are fully mapped, offering attractive alternative routes: the most significant is the North Wiltshire Rivers Route, a mostly traffic-free detour with some superb cycling along the Ridgeway.
So you can now use OSM as your free guide to cycling all the way from London to West Wales. Enjoy!
(And I took the opportunity to map the missing section of NCN 8, Lon Las Cymru, into Cardiff. Both this and the Pennine Cycleway, NCN 68, are also approaching OSM completeness.)