[Since the M98 is technically an express of the M4....]
Not quite. It was actually implemented as a branch of the M101, which is why it originally ran out of 100th Street Depot and uses 3rd/Lex instead of 5th/Madison. In fact, it was accompanied by a slight reduction in the number of M101 short trips from 125th Street. Otherwise, Transit would have needed to petition the Board of Estimate for a totally new route franchise. (BTW, that's how all the Queens A-suffix routes got started - the various companies just piggy-backed on existing franchises.)