Added 4-Sum Problem implementation

[kkn2k19]

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.79%. Comparing base (e499d3b) to head (a4d91c6).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5907      +/-   ##
============================================
- Coverage     66.80%   66.79%   -0.01%     
+ Complexity     4501     4500       -1     
============================================
  Files           611      611              
  Lines         16958    16958              
  Branches       3275     3275              
============================================
- Hits          11328    11327       -1     
  Misses         5182     5182              
- Partials        448      449       +1     

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[siriak]

We don't add leetcode problems