N gauge can go down to below 1ft radius so 4ft is wide enough for the main line. If you used the maximum radius of just under 2ft you would have only 2ft of straight track between the curves, so it might be better to have a tighter radius to get a longer straight (or nearly straight) section for your station and fiddle yard. A 4-car Voyager is about 2ft long, so about 3ft would give you room for points at either end of the station.
It is good to have cuttings or tunnels to hide the sharper curves as they do not look very realistic. Possibly you could put your fiddle yard on one of these curves with points on the straights either end?
I have a gradient of about 1 in 40 including a 90 degree curve of less than 18" radius. Farish diesels happily take 6ft freight trains up this, Farish 158s and 170s could do the same if it was prototypical for them, so obviously have no problems on their own. However my Dapol Voyager only just copes, this is known for having problems with gradients. The Dapol Super Voyager or forthcoming Farish Voyager/Meridian may be better.
For gradients you need to allow for a vertical curve rather than a sudden change between level and sloping - again Dapol Voyager is particularly sensitive to this. My vertical curves are about 1ft long and they need to be on straight track. If you avoid the Dapol Voyager you can probably go to 1 in 30 for the length of train you are likely to be able to fit, but even then I think you will struggle to get one line to climb over another one in the space you have available.