How does pokegen work, and what do I need to use it?

Question

Also, does it work on 4th gen, or just 5th gen?

Answer 1

PokeGen just gives you a Pokemon over GTS, any Pokemon you want. Any items, movesets, gender, etc.
You can use 4th Gen, too.
>PokeGen is a Pokemon data editor for Generation 4 and 5 games -PokeGen

There are a lot of guides for PokeGen. Here are some of them:
Guide
Guide
Video