The 1.4 update (the "Biotech" era) was one of the most transformative in the game's history. It introduced:
Cleaning up menu overlays and text descriptions in various languages. A Note on .rar Files
Correcting errors where certain scripts from the RimWorld Workshop would crash the game. Rimworld.v1.4.3531.rar
Added "Paint" options for buildings and expanded hair/skin color settings.
Introduced mechanics for rot stink, smoke, and various toxic gases. The Role of Version 1.4.3531 Rimworld.v1.4.3531.rar
Resolving issues related to mechanoid recharging and child-rearing mechanics introduced in the expansion.
This specific sub-version was a "hotfix" build. It didn't add new content but was essential for: Rimworld.v1.4.3531.rar
Faster loading times and better handling of large numbers of pawns and animals.