Slider turtles can grow up to 11 inches in size, and need materials to burrow in. In nature, they like wet, slimy areas. Four turtles in one tank is probably too many. You need to keep the water in the tank very clean, as their waste can lead to you contracting salmonella. You should feed them live goldfish, stick foods, lettuce and other green vegetables. A heat lamp in the tank, along with a rock where they can “sun” themselves would keep them happy.