I'm not really sure how these get allocated, but looking through some of my payment recipients, Revolut is also lowish in this block with 04-00-67, but also with leading zeros, Santander has 09-01-32 and 09-01-33, co-op has 08-92-99, Nationwide has 07-01-16, 07-08-06, 07-19-86.
That looks to me almost like potentially a bank can get a sort code block as large as just the first 2 digits, e.g. Nationwide which has been around for a long time and all seem to start 07-. But that doesn't explain then why Monzo and Revolut are both in the 04-00- block.
EDIT: it seems that sort codes are assigned by pay.uk [1] and Wikipedia has a surprising amount of information [2]. Most interestingly, originally banks were 2 digits long, and so each bank had their own 2 digit prefix, and the single digit banks had a zero appended, so e.g. NatWest was originally Westminster (6) and became 60-. There's a whole load of 04- sort codes for "Utility Banks" [3] and Monzo seems to have been the second such bank (BFC is before it, but 04-00-01 is unlisted, so maybe Monzo was the third).
[1] https://www.wearepay.uk/what-we-do/payment-systems/access-to...