[eknkc]

This makes sense. However the implementation detail of h_0(x) could also be foo(x) + bar(x) or whatever. At the end, you have a function that returns a fixed number of bytes.

I doubt any has function that we use today changes the algorithm halfway through its operation though. So your proposal might have a benefit in security by obscurity way.

[qwename]

I suppose I missed the assumption that h_0, h_1 and h_2 cannot be trivially decomposed into concatenations of other hash functions, whereas h_3 and h_4 can.

Also, there's no obscurity since h_1 and h_2 are the actual targeted hash functions, h_3 and h_4 are just to show that concatenation doesn't change the actual problem or make it harder.