Four sum - C# code

FourSum.cs

public IList<IList<int>> FourSum(int[] nums, int target) {
        if(nums == null || nums.Length < 4 )
            return new List<IList<int>>(); 
        
        var fourNumbers = new List<IList<int>>(); 
        
        Array.Sort(nums);
        
        var length = nums.Length; 
        for(int i = 0; i < length - 3; i++)
        {
            for(int j = 0; j < length - 2; j++)
            {
                var first = nums[i];
                var second = nums[j];
                var firstTwo = first + second; 
                var twoSumTarget = target - firstTwo; 
                
                var start = j + 1; 
                var end = length - 1; 
                while(start < end)
                {
                    var startVal = nums[start];
                    var endVal = nums[end];
                    var twoSum = startVal + endVal;
                    if(twoSum == twoSumTarget)
                    {
                        var list = new List<int>(new int[]{first, second, startVal, endVal}.ToList()); 
                        
                        fourNumbers.Add(list);
                        start++;
                        end--; 
                    }
                    else if(twoSum < twoSumTarget)
                    {
                        start++;
                    }
                    else 
                    {
                        end--; 
                    }
                }// end while loop                    '
            } // end for loop j
        } // end for loop i
        
        return fourNumbers; 
    }