I don't use the "r" word, but you finding 4 is a bit of a shock Jimbo - I think that's simply a matter of a lot of luck for you, they are scarce. Honest. The ones from the Harris collection currently sold are, to my knowledge, the first and last from that source.
I've certainly only encountered fewer than 20, including these that you mention, in 12 years of Mdina hunting.
I've seen/know of more crizzle stones.