Small "k" for kilo and big "b" for byte.
Edit: Turns out the official project's claim is indeed 4KB so the title is just bogus.
1kb = 1000 bits
1Kib = 1024 bits
1kB = 1000 bytes
1KiB = 1024 bytes
Obviously in practice nobody uses the decimal ones.
So, anyone who uses Windows has probably noticed at some point that "GB" "MB" and "KB" are the power-of-2 variants. Whenever I see one of those tech YouTubers say the formatted capacity of a 1 "TB" drive is 931 "GB" I just cringe. Most storage vendors use the power-of-10 definitions and Windows uses the power-of-2 definitions. 10^12 bytes / 2^30 = 931.32, so no, formatting your drive does not make 7% of the space just disappear.