Fill in the blank with the suitable Modal.

If I were a bird I _________ fly in the sky.

would

The correct modal to fill in the blank is Option 4: would.

The sentence is expressing a hypothetical situation, so the modal verb "would" is appropriate. The phrase "If I were a bird" indicates that the speaker is imagining themselves in a situation that is not real. The verb "fly" describes what the speaker would do if they were a bird.

The other options are incorrect because:

• "Should" is used to express obligation or duty.
• "Must" is used to express necessity or compulsion.
• "Could" is used to express possibility or ability.

Therefore, the complete sentence is:

"If I were a bird I would fly in the sky."