Wait a minute....
Shouldn't it be Pope Fourthus IV, Pope Fifthus V, and Pope Sixtus VI? (Let's conjugate these names properly.)
Can anyone tell I've been up all night?
LOL; it's a good joke, but the name Sixtus actually has little to do with the Roman habit of naming successive children after ordinal numbers (Quintus, Sextus, Septimus, for example). "Sixtus" (spelled with an "i", not "e", N.B.) is the Latinized spelling of a name that was originally rendered "Xystus", itself a slightly corrupted spelling of the Greek nickname Xystos, meaning "shaved." The fact that Xystus/Sixtus was the sixth successor of St. Peter probably contributed to the popular adoption of the Romanised spelling.