We prefer saving 2nd digit version number changes to more 'revolutionary' releases of the language.
Ok. then do not rename it from 4.0.x to 5.x when engine 1.1 is released. I just tried to avoid that there are 17 versions of a 4.0 version as it happened to 3, there are so many nice numbers (1-9) besides "0" and I don´t think there will be 9 major+revolutionary releases before php 5
as PHP is diveded up into language and functions and consists of both, major changes in them should (could) also trigger a 2nd digit change (eg. apache 2 API is supported too now)
2c
andré