The BK4 is a very popular brush with excellent reviews. It is the softest, and perhaps most floppy in my collection. The very opposite of scrubby. If you want to face lather using the tips of the brush in circular motions, move on. If you like to paint your lather on, you won't find a more luxurious feeling from many other brushes. It is far from dense and although approximately a 22mm knot, it seems much smaller because of its relatively low density. Personally, I prefer a firmer brush but plenty of people love it. It has the common silvertip ability to create great lather. Have a close look at the photos to give some idea of how it compares with the other Kents.
Knot 22mm approx