First off, remember that you don't have to take your helicopter on a level 4 raid as there's no one to rescue, so use that slot for an additional squad.
Second, the goal is to kill all the zombies before night time, and you get a bit more time to do so. You'll want to take speedy killing squads rather than slow ones (like the snipers). The best way I've found is to take a squad of barrier placers, a squad of sappers, and as many high leveled troops as possible (an upgraded artillery squad is especially useful and can replace the sappers).
Try to funnel the zombies with your barriers so multiple spawn points are forced to traverse down one path. Place your infantry there (if you have any), and place the explosive charges at the places where you think they will merge together and detonate them when the time is proper.