Explanation:

- Original Statement: "The main gate of the building was being guarded by gun-toting guards."

- Option 1: Gun-toting guards guarded the main gate of the building.

- This rephrases the sentence but uses simple past tense.

- Option 2: The main gate of the building were guarding gun-toting guards.

- The subject-verb agreement is incorrect here. "Were" should be "was."

- Option 3: Gun-toting guards were guarding the main gate of the building.

- This correctly rephrases the sentence in past continuous tense.

- Option 4: Gun-toting guards have been guarding the main gate of the building.

- This uses present perfect continuous tense, which alters the meaning slightly.