Hi Rob,
As noted above the number refers to the the degree to which the turnout track diverges from the straight track. The higher the number the more gentle the angle. Turnouts with larger numbers help accommodate higher speeds so #6 and higher are great for the main line. #4 turnouts are pretty sharp by comparison and are often used for sidings or spurs for low speed switching of cars. Larger locomotives can sometimes have derailment problems on smaller numbered model turnouts.
Ralph
Even our switches are pretty sharp compared to a real railroad. Count the ties from point to frog on a real switch, then compare it to a #6 or #8.