Here is the full list of all 23 POSTAL 4: No Regerts achievements.
Finish a mission with all 3 Dudes
Reunite with an old enemy, again
Flush after taking a piss in a toilet
Get all 10 petition signatures using the drag outfit.
As the Cat Dude, kill 10 chicks
Hit all barrels with an immigrant during the Border Smuggler errand
Beat all highscores of the Scooter Challenge
Reach maximum heat level with the police and survive!
Run over 50 people with the mobility scooter.
Make your homeless sign
Find the secret crack den!
Kill 100 pigeons from thrown cages
Kill 4 fast-food workers during the Mexicleaner errand with the FOURnicator
Kill all of the mountain men on Thursday
Install all bidets for Mike without killing anyone
Kill 7 people at the same time using Dude Eye
Beat the Fix Race errand by crossing the finished line with a fully bloodied scooter
Collect all the paintings in the game
Finish all rampages
Collect all Krotchy, Kunny, & Larry collectibles
Beat the game by only using kicks to open doors and kill people
Completed the story with no kills
Turn 10 toilets yellow with piss