The SHA-1 hash is 160-bits long, so with four base-64 digits, we only use
the first 24-bits of it. Using the first 25-bits is no different. No
padding is necessary. If you start converting the 160-bit hash to base-32
and stop after the fifth character, you've encoded the first 25-bits.
Ok, sorry. Thanks for the explanation. With that in mind, then I agree that a
lowercase 25-bit base32 (5 characters) is the best minimum recommendation for
hashes.