I think the /4 is not supposed to rest on the baseline like the /2, but float on its keel like a hovering fantasy ship. No a priori reason why that should be so, I guess... the /4 has always struck me as by far the most non-Latin shape among the numerals, and among the most non-Latin shapes in Latin typography overall.
Essentially, is is about respecting the proportions of the numeral. Whenever you have a triangular shape, you have a strong dependency between height and width, which in the case of the 4 means allowing the horizontal to float in order to maintain the counter shape at an appropriate width relative to the height.
Yeah it's a funny thing when it comes very close to baseline, but not quite, the impulse is to make it rest. I don't imagine it was ever intended to align in that way. This doesn't necessarily happen across all weights within a family either, so you'd have to force the issue, and then what would be the point?
I think PostScript, with its attention to hinting, especially alignment zones, has given an importance to the baseline, and created a semantic groove that present day type designers can easily fall into, imagining that things are supposed to be aligned.
Historically, the baseline was not quite so important (notwithstanding overshoot), except for aligning different fonts with one another. And when one considers that oldstyle figures are, by recognition and subsequently definition, ranging—all over the place—then the question might be rephrased: Is there any contemporary reason for sitting the crossbar of the 4 on the baseline?
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Historically, the baseline was not quite so important (notwithstanding overshoot), except for aligning different fonts with one another. And when one considers that oldstyle figures are, by recognition and subsequently definition, ranging—all over the place—then the question might be rephrased: Is there any contemporary reason for sitting the crossbar of the 4 on the baseline?