To learn how to get 4 crew see my guide for that achievement:
For this one just be on the lookout for potential crew members with the Revive ability. Since it's the best ability to have you'll usually want to pick them up anyways. If you've used one of their revives recently (which uses their halo) you'll need to find a new member with Revive or buy the Cabin III ship upgrade which gives a chance to replenish a used halo.
Here's a pic with all 4 halo-ed up crew members: