Preconditions:
Download Relief Terrain Package (https://www.assetstore.unity3d.com/en/#!/content/5664) from A$.

Steps to repro:

1. Open Unity 5.3.3f1;
2. Create new project and scene;
3. Import downloaded previously Relief Terrain Package;
4. Create terrain gameobject (GameObject -> 3D Object -> Terrain);
5. Go to 'Paint Texture' tab of Terrain component;
6. Add 4 different terrain-layer textures (e.g. Grass, Rocks, Ground, Snow);
7. Add Relief Terrain Engine component (Add Component -> Relief Terrain -> Engine-Terrain or Mesh);
8. Select ‘RTP_LODmanager’ gameobject in the Hierarchy panel;
9. In the Inspector scroll down and click ‘Recompile shaders for given feature set’ button;
10. Wait until shaders recompile;

11. Notice that after recompilation terrain shader is rendered stably without any errors (expected behaviour);
12. Save scene and project;
13. Close Unity;
14. Open Unity 5.4.0b8;
15. Open the same project and scene;

16. Notice that after the project is opened in 5.4.0b8 shader error is displayed:

Shader error in 'Relief Pack/ReliefTerrain-FirstPass': maximum ps_4_0 sampler register index (16) exceeded at Assets/ReliefPack/Shaders/ReliefTerrain/RTP_Base.cginc(664) (on glcore) Compiling Vertex program with DIRECTIONAL SHADOWS_SCREEN LIGHTMAP_OFF DIRLIGHTMAP_OFF DYNAMICLIGHTMAP_OFF RTP_PM_SHADINGPlatform defines: UNITY_ENABLE_REFLECTION_BUFFERS UNITY_PBS_USE_BRDF1 UNITY_SPECCUBE_BOX_PROJECTION UNITY_SPECCUBE_BLENDING SHADER_API_DESKTOP UNITY_TEXTURE_ALPHASPLIT_ALLOWED.

Also notice, that shader rendering terrain is broken (see attached actual.png and gif video).

17. Repeat steps 8-10 (shader recompilation).

Actual result:
Terrain shader is partially broken - texture is visible when viewpoint is far from terrain, however when scrolling in, it all goes purple (see attached gif video).
Regression introduced in Unity 5.4.0b8.

Expected result:
- Terrain shader should be rendered normally without fallbacks after upgrading project to 5.4.
- No errors should be shown after upgrading/recompiling shaders in 5.4.
Expected behaviour could be found in 5.3.3f1.

Notes:
- Reproducible in: 5.4.0b8;
- Not reproducible in 5.3.3f1;
- Reproducible on: OSX 10.11.3 and Windows 10;
- This bug could be package related;
- Opening project again on 5.3.3f1 after having received broken shader rendering in 5.4.0b8 leads to broken shader in 5.3 as well.