Codex Catachans for 4th edition Warhammer 40,000 was a slightly changed 3th edition codex released for free as a PDF on Games Workshops webpage. It is no longer available.
The Catachans were the go-to Imperial Guard army for several members of my gaming group in our heyday of Warhammer 40K gaming. I think that they just liked the muscular look of the models that gave off this Vietnam Rambo vibe. When this book came out, we actually used the jungle rules in many of our games and I thought the Ambush and Booby Trap rules (where you divided the game board into squares and secretly placed units and traps in them before the start of the game) were nice game mechanics that added a fun and interesting element to game play. I didn’t play Catachans, but I did have a large Kroot Mercenary army (the rules were in one of the issues of White Dwarf and were later reprinted in one of the Chapter Approved collections) that used a lot of the Catachan special abilities and there were some exciting games when you have two jungle-fighting armies battling it out, ambushing each other with booby traps going off. Sadly, as the book points out, this army (played as a pure Deathworld army) is disadvantaged in games where the jungle rules (or terrain) is not in use (my Kroot army suffered as well), but the models still served to be part of a standard Imperial Guard army (with an option to pay points to make them Deathworld Veterans).