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:
- Signing a form isn’t much more than signing a form is a formality.
The first part of the comparison is a noun: the act of signing a form. The second part of the comparison is a clause: an statement about signing forms. You cannot compare an action to a statement.
Sentence 5 has the same issue.