Hi again, plbab;
Radius is one of the most important things in N-Scale. The larger the radius, the smoother the ride.
I run about 20" on my turnarounds. This is mainly for the passenger trains I like to run with their 85' cars. Also my Bachmann 4-8-4 does better on larger radius curves.
How well a train goes around a curve is also related to speed. I very slow train, as in a switcher, can take a tighter curve, say 9½". This means that you are really going to have to go with flex track, since the largest pre-formed curve Atlas produces is an 11". They also produce a 19" radius track, but it is hard to find, at least in my area.
Just keep in mind what you plan on doing with the layout and remember, even though you have 4' end, you'll have to leave an inch or two on the outside, thus giving you only 46" or 44". Also keep in mind that you will be bending over this layout, and it is impractical to have too large an area because you can only reach in 18" or so, usually.
If you are building a walk-around layout, this will not be a problem.