Check out this detailed graphics comparison between DiRT 4 versus DiRT Rally. Both games look great, each with it's own strength. Both games set to ULTRA settings.
My own analysis about both games graphics. Some say DiRT 4 looks downgraded compared to DiRT Rally. The thing is that DiRT 4 has a different art style. It's more focused on real life lighting and it's not as vibrant as DiRT Rally, which has increased color vibrance, contrast, the shadows are way darker, etc. The motion blur also is more heavily used in DiRT Rally compared to DiRT 4. Cars have more realistic reflections in DiRT 4. And the lighting of the level has a bigger impact on the car paint job. Dusk, cloudy, foggy, rainy. It reflects the sky alot better than in DiRT Rally. In DiRT Rally it's mostly just that vibrant red, blue,yellow paint, no matter the weather setting.
But the trees in DiRT 4 look worse. They look less smoother than in DiRT Rally and the atmospheric fog surrounding them is also missing mostly. The exhaust particle effect is also worse looking in DiRT 4.
Truth is that some levels look better in DiRT 4 due to superior more life like lighting, while some levels look nicer in DiRT Rally.