Watch the official “What if Jack Lived?” clip from This is Us Season 4. The NBC drama series stars Milo Ventimiglia, Mandy Moore, Sterling K. Brown, Chrissy Metz, Justin Hartley, Susan Kelechi Watson, Chris Sullivan, Logan Shroyer, Hannah Zeile and Niles Fitch.<br /><br />This is Us is now streaming on Peacock.<br />
