A series of (hopefully non-controversial) tweaks to some common color contrast issues that V4 inherited from V3.
Some of these tweaks aim for the full 4.5:1 color contrast ratio recommended by WCAG 2.0, while others (notably the success, info and warning tweaks) go for the slightly more relaxed 3:1 (which is really for large text, so still not ideal in all situations...but a slightly more achievable goal, particularly for vibrant colors).
Not all possible combinations have been tried out, but most tweaks seem to move towards a more positive result judging purely by the documentation examples.