Extend from 3Sum. Add another for loop. Slow Version

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> res = new ArrayList<>();
 
		if (nums == null || nums.length < 4)
			return res;
		int len = nums.length;
 

		Arrays.sort(nums);
        
        for(int i=0;i<len-3;i++){
            if(i>0 && nums[i]==nums[i-1]){
                continue;
            }
            for(int j=i+1;j<len-2;j++){
                if(j>i+1 && nums[j]==nums[j-1]){
                continue;
            }
 

                int left=j+1;
                int right=len-1;
                while(left<right){
                    int sum=nums[i]+nums[j]+nums[left]+nums[right];
                    if(sum==target){
                        res.add(Arrays.asList(nums[i],nums[j],nums[left],nums[right]));
                        while
 
(left<right && nums[left]==nums[left+1]){left++;}
                        while(left<right && nums[right]==nums[right-1]){right--;}
                        left++;
                        right--;
                }
                else if(sum<target){
                    left++;
                }
                else{
                    right--;
                }
               
            }
            }
        }
        return res;        
    }
}

Fast Version

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> res = new ArrayList<>();
 
		if (nums == null || nums.length < 4)
			return res;
		int len = nums.length;
		Arrays.sort(nums);
        
        for(int i=0;i<len-3;i++){
            if(i>0 && nums[i]==nums[i-1]){continue;}
            if(nums[i]+nums[i+1]+nums[i+2]+nums[i+3]>target){continue;}//add
            if(nums[i]+nums[len-3]+nums[len-2]+nums[len-1]<target){continue;}//add
            for(int j=i+1;j<len-2;j++){
                if(j>i+1 && nums[j]==nums[j-1]){continue;}
                if(nums[i]+nums[j]+nums[j+1]+nums[j+2]>target){continue;}//add
                if(nums[i]+nums[j]+nums[len-1]+nums[len-2]<target){continue;}//add
                int left=j+1;
                int right=len-1;
                while(left<right){
                    int sum=nums[i]+nums[j]+nums[left]+nums[right];
                    if(sum==target){
                        res.add(Arrays.asList(nums[i],nums[j],nums[left],nums[right]));
                        while(left<right && nums[left]==nums[left+1]){left++;}
                        while(left<right && nums[right]==nums[right-1]){right--;}
                        left++;
                        right--;
                }
                else if(sum<target){
                    left++;
                }
                else{
                    right--;
                }
               
            }
            }
        }
        return res;        
    }
}