There have been many editions of the Ilford Multigrade filters. The biggest improvement came when they were made so that grades 00 through 3-1/2 all required the same exposure and grades 4 to 5 required twice as much. That came before Multigrade IV and lasted well after its introduction. Before that, exposure varied with each filter. Since then, they have probably changed again. Probably the paper has, too.
The Multigrade filter numbers do not really correspond to paper grades (well maybe loosely). They just provide a useful sequence of contrasts. Different papers also respond quite differently to the filters. And filters differ from one manufacturer to another (Kodak is different from Ilford, although either set can be used with Multigrade). Ilford has probably engineered its current set to provide the best range of contrasts it can reasonably get from its current paper. They will work differently and probably better than the older filters, but the older filters can still be used with almost the same results, given some adjustments.
(Of course, I'm just barely old enough to remember when the high contrast Multigrade filter was greenish yellow and the low contrast one was magenta. (1950s).)