Platinum Difficulty Rating - 5/10
The bulk of the Single Player trophies in this list are related to the Tour mode, and most of them can easily be achieved through natural progression.
There are only 60 events in the Showdown Tour this time round, which is a drastic cut from the previous DiRT games. The "GOLD!" trophy, for completing the Showdown Tour and winning Gold in every event still isn't that difficult to achieve, and there are a host of other progression based trophies tied to winning at bronze and silver levels too, as well as beating each of the 4 tiers in the Tour mode, so there is always something to spur you on.
Sticking with the gold theme, the "Max Power" trophy, which requires you to fully upgrade all vehicles, is the other gold trophy in the list, and requires you to kit out every vehicle to maximum specifications. You can earn the majority of the money needed to upgrade everything through just natural tour progression, though you may also need to venture online to scrape together the remaining money needed (or just replay the higher reward Tour events over and over).
Even though there are a considerable amount of throw-away trophies, the "Wrecker" trophy, which requires you to achieve a 5x wrecking streak in Rampage, and "Hitman", which requires you to achieve all hit types in Rampage, will test your skills, and possibly require a bit of luck in the process.
You also need to collect all 100 hidden packages in Joyride mode, as well as beat 100% of the Yokohama and Battersea missions too, which aren't only a touch grindy, but frustrating at times aswell.
Venturing online, and the "Showdown Superstar" trophy, which requires you to reach fanbase level 30 online, makes a return, and has the exact same fanbase target as DiRT 3 (250,000 fans). In the exact same way too, it is also incredibly lengthy and will easily take somewhere in the region 30-40 hours alone.
The "Running the Show" trophy requires you to dominate a friend in challenge mode (10 wins on the bounce), which is no minor task, though as long as you make sure you don't post any challenges to them, you should be able to accumulate this over time, as long as your friend keeps on posting challenges to you.
There are a couple of trickier online trophies, including "Loot Carrier", which requires you to hold the loot for 2 minutes in Smash and Grab, which is a tall order when you take into account the games are only 3 minutes long, and "Showdown Haulage Co.", which requires you to drop the flag off 5 times in a round of Transporter.
Generally speaking though, this is a pretty easy Platinum to achieve. The Showdown Tour shouldn't take any longer than 15-20 hours to complete in full, and given the fact you can achieve the vast majority of the single player trophies on the way to obtaining a gold trophy in every event, the need for a mop up afterwards probably won't be hugely necessary.
Even though the Multi-player trophies do their best to step up the challenge a notch, there still isn't anything overly difficult, and if you can get over the fairly steep completion time (About 70-80 hours in total), you've netted a pretty easy going Platinum.
Notable Trophies -
Showdown Superstar
Reach Fanbase Level 30 Online