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?

RFC9999 obviously should be to propose RFCs having 5 digits

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.