To supplement cruthers' fine answer:

Sentences 4 and 5 are impossible for anyone to understand.

The comparison is made with "*isn't much more than*". Here's the structure, with the two items being compared in bold:

4. **Signing a form** isn’t much more than **signing a form is a formality**.

The first part of the comparison is the act of signing a form. The second part of the comparison is the degree to which signing a form is a formality. You cannot compare **an action** to a **degree of formality**.

Sentence 5 has the same issue.