Hi iL Dottore,
You are correct it is 4mm scale.
The track base is plywood with 1/8in cork on to which the track is laid.
The columns hill side is cardboard supported with balsa strips.
The viaduct columns are cardboard turned into boxes then covered with Slater's embossed plasticard and the arch insides are a single sheet of plasticard cut to the right length then curled up and glued into place.
Timewise, taken 3 or 4 months on and off although the lefthand half was built in breaks from revision this week so 10 hours for that half I'd say.
Hope that answers all your queries,
DRoe96