This may have something to do with access points. The proposed BxM4 would access it from Jerome Avenue, whereas the current BxM2 does so at 230th Street (for reference, some BxM3 trips do so at what is effectively 240th Street). Moreover, the BxM4 currently follows Grand Concourse, Bainbridge Avenue, and Jerome Avenue, a route that can hardly be considered speedy.
Whether or not the proposed alignment is actually faster remains to be seen, especially without bus/HOV lanes. (For what it's worth, I doubt it, and I have a hard time imagining that the highway would even get those lanes, given what's around it for most of the relevant stretch.)