Added in one more python solution. Using defaultdict.

problems/0454.四数相加II.md

@@ -141,7 +141,24 @@ class Solution(object):
 
 ```
 
+```python
+class Solution:
+    def fourSumCount(self, nums1: list, nums2: list, nums3: list, nums4: list) -> int:
+        from collections import defaultdict # You may use normal dict instead.
+        rec, cnt = defaultdict(lambda : 0), 0
+        # To store the summary of all the possible combinations of nums1 & nums2, together with their frequencies.
+        for i in nums1:
+            for j in nums2:
+                rec[i+j] += 1
+        # To add up the frequencies if the corresponding value occurs in the dictionary
+        for i in nums3:
+            for j in nums4:
+                cnt += rec.get(-(i+j), 0) # No matched key, return 0.
+        return cnt
+```
+
 Go：
+
 ```go
 func fourSumCount(nums1 []int, nums2 []int, nums3 []int, nums4 []int) int {
     m := make(map[int]int)