Benchwork I went with plywood top with 4" of foam. Reason being I want to do sub surface features, like a pond, streams the track crosses and an abandoned rail line my layout 'crosses over'. Never used it before, will be interesting to see how this plays out.
I like your plan to go with plywood topped by four inches of foam. That's how my layout is constructed because I wanted to have realistic terrain above (hill-tunnel) and below (river valley) track level.