The current Multigrade filters allow the same exposure times for grades 00 to 3.5, and then double the exposure time for grades 4 to 5. That is, I believe, the reason they are not simply yellow at the low end and magenta at the high end. Older filters like that can be used, but would give varying times for each grade. I inherited a set of Kodak Polycontrast filters like that. And I do believe the current multigrade filters are optimized for multigrade IV paper.