I'll be on it late September and expect virtually no one to be on it at that point. In the summer months, I would expect to find someone walking the lesser routes. But off season, it's unlikely. So I wouldn't worry about that. Also people are kinda friendly in Spain.
As to how long it takes you, I think 4 days is possible if you start the Leon to La Robla stage day one and aim for Cabanillas. Then do Cabanillas to Buiza on the next day. On Gronze, stages 3, 4 and 5 are all rated 4-5 star, presumably because you are crossing the Picos at that point. My own plan is maybe to do stage 3 and go as far as Benduenos on the same day if I can, then try and hit Oviedo the next day, but will probably stop at Mieres and that would allow me to hit Escamplero the next day. But I know my limits and plans change. I personally dislike cities and staying in cities is generally more costly than staying outside of them. I have also been to both Leon and Oviedo several times in the past, so I will probably swing by each Cathedral and get a stamp, then hit the Camino route. In Leon I may not even do that. The Cathedral area is too busy.
YMMV.