Does the major version number (4.x vs 5.x) mean anything?

No. The major version number is incremented when the number after the dot starts looking “too big.” There is literally no other reason.

https://www.kernel.org/category/releases.html

Also,

Does the odd-even number still mean anything?

A long time ago Linux used a system where odd numbers after the first dot indicated pre-release, development kernels (e.g. 2.1, 2.3, 2.5). This scheme was abandoned after the release of kernel 2.6 and these days pre-release kernels are indicated with “-rc”.