Which motor is slower than (and can be swapped with) a 28BYJ-48 stepper motor?
You are probably out of luck, unless you are really, really lucky.

Your motor is a 4-wire unipolar motor, which probably takes 64 steps per revolution (the small, cheap ones do that). It also has a gear train which reduces 64 step/rev to 4096 step/rev. In other words, it has a reduction ratio of 64:1.

What you're looking for is a similar motor with a reduction ratio of 256:1. And, sorry to say, I suspect you won't have much luck. You're getting down to mechanical gear size limits, especially since I assume you need a drop-in (mechanically identical package) replacement.

You can Google on "geared stepper motor". Who knows, you might get lucky.