一个简单的区分办法：如果much后面有被修饰的名词，且其后的as从句在意义是修饰其前名词的定语，则为定语从句，否则便是状语从句。

所以，你的第一个句子中的后面一个 as 引导的是定语从句，其他三个为状语从句。

再补充一下，比较状语从句通常为省略句，比如：

1. I'm sorry I haven't done as much as I should.

=I'm sorry I haven't done as much as I should do.

2. I ate as much as I could.

= I ate as much as I could eat.

3. Some doctors are paid almost twice as much as the nurses.

=Some doctors are paid almost twice as much as the nurses are paid.