Yes, hardware support goes by major version. OpenGL 4.2 hardware will support all OpenGL 4.x.
Generally true, except for Intel and Apple?
It is about versioning convention in opengl not about implementations. Opengl hardware requirements grows only when major version increases. In another words, if hardware does support any version of opengl 4.X it's capable to support any other 4.Y
BTW Intel supports opengl4 under winows for some of their 3D accelerators and rumors are, Apple is working on OpenGL 4 drivers