A character within 6" of a unit with a Look-Out may re-roll failed "Look Out Sir!" tests, and characters in the unit may use the rule even if they are below 5 models.
Source: Warhammer Armies Project: Unofficial 9th Edition
A character within 6" of a unit with a Look-Out may re-roll failed "Look Out Sir!" tests, and characters in the unit may use the rule even if they are below 5 models.