Steps to repro:

1. Open Unity;
2. Add Terrain component (GameObject -> 3D Object -> Terrain);
3. Go to any Inspector's tab of Terrain component (e.g. Terrain Settings);
4. Change value for any scale bar, checkbox or dropdown list;
5. Either press cmd(ctrl) + Z or select Edit -> Undo.

Actual result:
All values of the Terrain settings stay the same, which creates inconsistency within the product. For example, in the Camera component any twekable property (projection, field of view, clipping planes) could be reverted by undo.
See attached actual.png screenshot.

Expected result:
After executing Undo last changed Terrain component's value should be reverted to its previous value.

Partial workaround (only for Terrain Settings tab) is to reset component by clicking on cogwheel

Notes:
- Reproduces in 5.4.0b5, 5.3.1p1, 5.2.4p4, 5.1.3f1;
- Reproduces for both Windows 10 and OSX 10.11.3;
- Undo works fine for Terrain Collider component.