As far as I know, it's just a small difference in tip opening, according to Vandoren's website (https://vandoren.fr/en/saxophone-mouthpieces/), the A27 is 1.65mm, A28: 1.63mm, both with a medium short facing. The AL4 is 1.631, MS, so yes it corresponds to the A28. I played an AL4 but really didn't like the combination of open tip (in classical terms) and short facing, so switched to an AL3 (the AL5 wasn't around back then), and have been happy with it ever since.
I mean, they have the same tip opening and facing and they both have a round chamber. But the internal design is quite different, the baffle is bigger on AL4 and the AL4 has a "darker" sound.
AL4 is more reed-friendly, while with A28 found a good reed is difficult.
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