I wasn't able to access a working archive page of the end of the 4th season, but on the same day, 2 "Where Are They Now?" episodes have been broadcasted as visible in this page:

• At 9pm, the one for season 3. We need to list this one as a special episode as we can't number it as a season 3 episode at the end of the 4th season.
• At 10pm, the one for season 4. This one is an episode of the 4th season as this is also confirmed by this page. I quote: "The fourth season of “60 Days In” aired between January 1 and March 26, 2018.".