This is a nice overview.
I did some quick BTZS tests with Kentmere Bromide last year and couldn't tell the difference between the grades 3 and 4 curves. Was it just me/my paper batch or are these two really the same?
I've gone though a 25 sheet package of Grade 4 and believe its more contrasty than grade 3 is. I can't tell why you got the results you did.