Which is a valid example of denying the consequent?

Exceptive Syllogism: Valid Inferences from Connected and Disjunctive Conditionals

• If square then four-sided; four-sided; so square
• If square then four-sided; not square; so not four-sided
• If square then four-sided; square; so four-sided
• If square then four-sided; not four-sided; so not square