DiRT Rally really is excellent and yes, it can be played reasonably well with a controller (I’m sure it’s better with a wheel, but who has the cash and/or space for that kind of thing?)
I haven’t played enough DiRT 4 yet to draw any conclusions, beyond the fact that it’s evidently the easier of the two games. Mind you, there are plenty of difficulty settings and I’m sure you can make it as challenging (perhaps not as satisfying?) as Rally, but by default it demands less of you. Of course, if you don’t have a wheel, that might be a good thing. There do seem to be a larger variety of event types (Rally is very concentrated on the traditional timed point to point race, with just a little Rallycross thrown in, whereas DiRT 4 has a range of disciplines).
My general sense is that DiRT 4 is wider, shallower, and probably more approachable than Rally, so it broadly depends on what you want. I think the world of Rally, but it’s all about doing one thing very (VERY) well. DiRT 4 is a more generalist “rally” racing game from what I can tell.