The major cause of `October Heat` is high temperature associated with high humidity. This means that during the month of October, there is a combination of both high temperatures and high humidity levels, which leads to hot and uncomfortable weather conditions. Option 1, hot and dry weather, does not accurately describe the major cause of `October Heat` as it does not take humidity into account. Option 2, very low velocity winds, is not directly related to the cause of `October Heat`. Option 3, low pressure systems over the Indo-Gangetic plains, is also not the correct answer as low pressure systems may contribute to weather patterns but do not directly cause `October Heat`. Therefore, the correct answer is option 4, high temperature associated with high humidity.