Perhaps you have noticed the weird version numbers of TeX. When TeX reached version 3 Knuth didn't want TeX to evolve further into new versions like most software does. Instead he wanted it to converge to a final and bug-free TeX. Hence the version number now converges towards the mathematical number pi. After version 3 followed version 3.1, then 3.14 and the current version is 3.1415926.
In a similar fashion Metafont has a version that converges towards the mathematical number e (currently 2.718281).
Based on this, the answer to your question is that Knuth doesn't want TeX to evolve into TeX version 4.
