El Nino is a climate phenomenon that occurs at irregular intervals, making statement I correct. It is characterized by the warming of ocean waters in the Pacific Ocean along the equator, which makes statement II correct. This warming of the ocean waters disrupts normal weather patterns and has a significant impact on global climate.

However, statement III is incorrect since El Nino actually carries more saline water. During El Nino, the usually cold and nutrient-rich waters off the coast of South America become warmer and less saline. This change in water conditions affects the marine ecosystems and fisheries.

Statement IV is correct. El Nino is associated with the atmospheric equivalent known as the Southern Oscillation. The Southern Oscillation refers to the see-saw pattern of atmospheric pressure between the eastern and western Pacific Ocean. When El Nino occurs, the atmospheric pressure in the western Pacific is higher, while in the eastern Pacific, it is lower. This change in atmospheric pressure affects the trade winds and weather patterns.

In conclusion, the correct answer is option 4 as it includes the accurate statements about El Nino, while option 3 is incorrect because it does not include the correct statements about El Nino.