I think if it's a study of light, then the first two pictures are possibly more instructive than the last two. The first one has deep shadows and bright light at the end of the alley, which places you in the position of exposing for shadows or for light. The question is, what is more important? Is there something more interesting in the light? Then you expose for that and allow detail to be lost in the shadows, let them be a sort of "silhouette" to frame the subject. Or perhaps the shadows hold what's interesting and you have to decide if it's important enough to blow out highlights in order to preserve detail in the shadows.
The first shot is not particularly interesting in terms of the subject, but as I said, it was a good picture to take to study the light contrasts. In this case, you did a good job of balancing shadows and bright spots. That's good if that is what you are after. Did you bracket (take exposures above and below the middle exposure)? Those other exposures are just as instructive as the one you eventually settled. Those will help you see the different effect of exposing for either light or shadows. Sometimes 'balanced' isn't always the most interesting use of light.
The second one isn't as dramatic a difference, but there's a nice gradient of light that was captured, and you get more of an idea of the quality of the light as well.
In the third one, you can see that the morning light isn't nearly as harsh even when it's falling directly on a subject, though you only get a little hint of the kind of shadows it can throw because there's only that small shadow in the middle. And in the fourth one, it looks like the light is very diffused, like it had clouded over, so the picture feels a bit dull.
I think you're on the right track when you go out to take pictures with very specific things in mind. Keep paying close attention to the light - and don't bin the ones you think are too dark or too light, at least not as first. Especially the ones that are in mixed light as in the first and second shots you posted here. They might not be good pictures but they'll help you understand different effects when you compare them to exposures that are more successful. And you're also right when you say the best way to learn is by doing!
