It sounds as if you established the correct time, presumably with test strips but without filtration and then dialled in Y&M to give you the correct grade.

If 4 secs was correct without filtration then you dialled in filtration using the same time I am surprised that the final print and test strip didn't appear different. Any filtration will increase the time required to get the same looking print as the test strip.

If 4 secs looked right on the test trip without filtration then with filtration I'd have expected the print to be underexposed and look lighter.