LOL the whole point in Pentium was that it came after the 486 and had the word 5 in it, to imply it's the 586. Pentium 5 would be ridiculous, I mean it'd be like 5ium 5. They should have moved onto the Hexium and Septium ages ago instead of Pentium II, III, IV etc.
Then we still have Octium, Nonium, Decium, Can't think of 11prefix-ium, Dodecium, etc.