@Benoit W Did you ever try to play them before you decide to buy it some? This will definitely give you a better view to be able to decide.
If you are on PC then go for the Dirt 4. The game is much more polished and fun than the classic Dirt Rally. You may have to choose based your preference obviously. But if I were to be in your place I'd choose Dirt 4.
In the end of the day, despite the necessary technical aspects and everything else... It's a matter of personal taste.
The reviews serve to give you a basis of what to expect, not to set your preferences. But it's a shame that not everyone can separate things properly.
I would say Dirt Rally has better graphics with many more surfaces and different texture types than Dirt 4. The cars are more realistic and the crowds are better, also the rain and snow look better as well in Dirt Rally. The sound is better with the gravel hitting the car, and it has better water splashes as well.