>>149185004
S4 was a big bump in quality, I think most of the characters got improved models, the environments became more detailed and the lighting got much better so the characters look more like they belong in the environments. Compare this image to the earlier season screencaps above, it's like night and day.

I have to wonder if it was always planned to end after S4 or they thought it would continue, because it seems strange to make such a big investment in improving the quality of the show just for one season.