Simple Java Solution, 18ms, O(n^3), HashSet (100% beat)
class Solution {\n    public List> fourSum(int[] arr, int target) {\n        \n       Set > set = new HashSet<>();\n        Arrays.sort(arr);\n        \n       
class Solution {\n    public List<List<Integer>> fourSum(int[] arr, int target) {\n        \n       Set<List<Integer> > set = new HashSet<>();\n        Arrays.sort(arr);\n        \n        for(int i=0; i<arr.length-3; i++){\n            for(int j=i+1; j<arr.length-2; j++){\n                int left = j+1;\n                int right = arr.length-1;\n                \n                while(left<right){\n                    int sum = arr[i]+arr[j]+arr[left]+arr[right];\n                    \n                    if(sum==target) {\n                        set.add(Arrays.asList(arr[i],arr[j],arr[left],arr[right]));\n                        left++;\n                        right--;\n                    }\n                    \n                    else if(sum<target) left++;\n                    \n                    else if(sum>target) right--;\n                    \n                }    \n                    \n            }\n                               \n            }\n        \n        return new ArrayList<>(set);\n        }\n}\n
['Two Pointers', 'Ordered Set', 'Java']