Gene,
The turnout number in a model is a relationship to the angle of the frog within the switch and the corresponding radius of the curved rail that turns away from the straight rail.
Thanks to a chart in a book by John Armstrong, pg.42 of Creative Layout Design, I can give you the following for N scale:
#4 = 45 degree curve = 132' prototype radius = 10" radius in scale
#6 = 14 degree curve = 408' prototype radius = 31" radius in scale
#8 = 7 degree curve = 800' prototype radius = 60" radius in scale.
As you can see, the #4 is quite sharp and should be limited to 2 axle locos and short cars. The speed rating in prototype for a #4 is 20mph where as in #6 it is 35mph. Still sharp actually by prototype standards. A #8 speed limit is 45mph.
Just in case you want to know, a turnout with a 1 degree curvature = 5729' prototype radius = 430" radius in scale has a speed limit of 100mph.
If you have the book there is more to it.
B-) & :x