BAK-4 Versus BK-7 Prisms
BAK-4 is a high density glass of better optical quality than BK-7. With BAK-4, if you hold binoculars away from your eyes and up to the light you can see the circular exit pupils in the eyepieces. The less expensive BK-7 prisms have squared-off, non-circular exit pupils.
The prism is alike: Bak4 prism costs more than Bk7 prism - Bk7 prism is fine to most of the users, but, to some "picky" users, Bak4 prism is a must. The difference is the quality of glass and anti-reflective coatings.

The design of the prisms can affect the quality of the exit pupil. The BK7 prism typically found in lower priced binoculars produces an exit pupil with shaded (blurred) edges, whereas the BAK4 prism has a perfectly round exit pupil. Whilst the BK7 prism design would look no different to the BAK4 in daylight, as the light levels drop, and the eye pupil expands you start to observe the effects of the shaded regions as the image quality drops and becomes prone to chromatic aberration around the periphery of the image.
See the illustrations bellow