For the most part, you'll be able to get away with using Textures for Miki 3 on Miki 4. The differences in Miki 4 and Miki 3 are not uv based but more so, how the shape flows in the morph. Miki 4's brow is different from Miki 3's in shape and so you may find that large eyebrows are now thin and that sort of thing. That said, 9 times out of 10, you won't have an issue with textures.

 

As for morphs for Miki 3. The poly count isn't changed, just the grouping, so in theory, you should be fine.