What is the radius of Bachmann #4 switch?
The #4, #5, and #6 switches don't actually have a "radius". The diverging track is actually straight and, measuring from the frog, diverges 1" from the through track at 4", 5", and 6" distance respectively.
That said, the diverging angles for each are: #4 - 15 deg, #5 - 12 deg, and #6 - 10 deg. So a #4 turnout could substitute for a 1/2 section (15 deg arc) of full (30 deg arc) 15" or 18" radius curves. But due to the staight diverging leg, doing so will widen the curve a bit more than using an actual 1/2 curve section, or "standard" switch with a curved diverging leg.
Len