Add 4-Sum Problem

[emmariescurrena]

• [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 64.22%. Comparing base (213fd5a) to head (7c9738b).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5788   +/-   ##
=========================================
  Coverage     64.21%   64.22%           
- Complexity     4216     4217    +1     
=========================================
  Files           588      588           
  Lines         16426    16426           
  Branches       3170     3170           
=========================================
+ Hits          10548    10549    +1     
  Misses         5448     5448           
+ Partials        430      429    -1     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[siriak]

We don't add leetcode problems