Here is the full list of all 48 DiRT 4 achievements. It takes around 60-80 hours to unlock all of the achievements on Windows.
Complete back to back Weekly Community Events
Win the Historic Legends Series
Achieve 10 Gold medals in Joyride
Destroy 100 blocks in 60 seconds in Joyride
Set a Gold medal time in a Time Attack challenge without hitting a penalty marker
Earn the International Rally H-A licence
Win the Triple Crown
Finish a Stage in sunny conditions in Community Events
Finish in the Top Tier of a Delta Daily in Community Events
Repair your car on Stage and win the Stage
Complete an Event in Jam Session
Win the FIA World Rallycross Championship
Qualify in the top spot in a Rallycross Event in Career
Win the Landrush World Series
Complete a Stage or race with the Fearless Preset active
Win the Global Rally Series
Complete all 12 Stages of a Monthly Event in Community Events
Complete all of the Advanced Driving Techniques at the DiRT Academy
Hire a fourth Engineer onto your Team
Earn the International Off-Road C-1 licence
Earn the International Off-Road C-3 licence
Take two Joker Laps and win a Rallycross race
Set a time in every challenge in a single Joyride Chapter
Upgrade three Facilities to Grade A quality
Own a vehicle equipped with all Grade A parts
Earn the International Rally R-1 licence
Earn the National Stadium Pro-1 licence
Win a Stage at night
Triumph in the face of adversity
Finish three sectors of a Stage with a flat tyre
Earn the International Rally H-C licence
Complete 25 Events in Pro Tour
Get promoted to a new Tier in Pro Tour
Finish in the Second Tier or higher in a Daily in Community Events
Call in the Chief Engineer's recommendations during a Service Interval
Qualify for an A-Final in a Landrush Event in Career
Win a Stage in Headcam
Get a bad case of the Mondays
Earn the International Rally R-3 licence
Earn the National Stadium Pro-3 licence
Get an overall win in the lowest eligible Vehicle Class in a Rally Event in Career
Win your class in a multi-class Event in Career
Buy a vehicle from the Classifieds
Earn the Fearless Bonus in a Stage or race
Finish in first place in a Career Event
Complete the Welcome Event
Complete a clean Stage or race
Cleanly land a jump longer than 65m