RFCs have four digit numbers. This will likely change within a month or so; RFC 9945 was recently assigned so it won't be long. I wonder what RFC9999 and RFC10000 will be?
I'm probably neither creative- nor connected-enough to do it myself, but somebody should see to it that either RFC9999 or RFC10000 is funny as hell and lands on April 1st.
RFC9999 obviously should be to propose RFCs having 5 digits