Well, first of all, you're working with pre-launch build photos for both games here, so they're not the most representative that they can possibly be of each game.

Secondly, what Halo 4 environments? In that picture, I see a bit of foliage on the ground, some Covenant aircraft zooming across the sky, and a giant picture of a jungle dominating the background. The seam where the actual geometry ends and the picture of the mountain jungles begin is quite jarring, even just in this picture. Sure, the lighting is really, really nice looking - I'll give it that - but it just falls apart for me when I spend any amount of time looking beyond the boundaries of the level.

I also have to rebut your statement about 343 deciding to put "the actual graphics in gameplay" instead of "wasting resources" by adding some environment details. We could do an in-depth analysis, but I don't think it's necessary - I think we've probably both got enough experience in each game to know that I'm telling the truth when I say a lot of aspects of Halo: Reach simply looked better than Halo 4. H4 traded a lot of graphical fidelity for a pretty lighting system. Its textures sure don't hold up, though. Even without zooming in on a character's body, you can see how muddy and pixelated everything looks in H4. This is a common issue on arguably every single character model. If you look down, you'll even notice your own character's leg textures look horribly underdetailed in first-person. Geometry textures commonly share similar issues, especially in levels like Reclaimer. Rock structures just look drab and muddy and pixelated. So it's no wonder, then, the skybox is just a giant green picture.

Point is, it's quite obvious to see in comparing H4 with Reach that 343 could've put a bit more stock in aspects of graphical fidelity other than lighting, which clearly took up a lot of their budget. Reach just doesn't really have a dynamic lighting system, but even while playing it today, I'm so immersed by a lot of gritty, realistic art and all of these convincing character models to notice too much. When I kill an Elite and go to examine it up close, the textures don't look like poop. And since this thread is about skyboxes, I guess I should mention that almost every single Reach skybox looks amazing - none of them look like just muddy and pixelated images of green skies or flat pictures of unmoving clouds. In fact, the skyboxes of Reach are quite dynamic - as I've said above, many of them stretch far into the distance with actually-rendered, moving 3D geometry. And that's in 2010. I'm quite sure it can be done nowadays.

I just find it hard to believe 343 couldn't find a happy medium. While the immense lighting is really nice, sub-HD textures that fall apart even without zooming on them just aren't nice. On the other hand, while rendered 3D geometry throughout all the skyboxes is really nice, a lighting system that doesn't pop really isn't nice. I just think there's a happy medium in there somewhere - a place where a beautiful, visually dynamic skybox can live alongside an advanced lighting system.