CompatHelper: bump compat for Compat to 4, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the Compat package from 2.1, 3 to 2.1, 3, 4.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[codecov]

Codecov Report

Merging #94 (eb2b481) into master (5f98d39) will increase coverage by 0.04%.
The diff coverage is n/a.

❗ Current head eb2b481 differs from pull request most recent head 052312a. Consider uploading reports for the commit 052312a to get more accurate results

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
+ Coverage   19.29%   19.34%   +0.04%     
==========================================
  Files           7        7              
  Lines         456      429      -27     
==========================================
- Hits           88       83       -5     
+ Misses        368      346      -22     

Impacted Files	Coverage Δ
src/Inputs/PWscf.jl	90.78% <0.00%> (-0.57%)	⬇️
src/Inputs/PHonon.jl	0.00% <0.00%> (ø)
src/Outputs/PWscf.jl	0.36% <0.00%> (+0.02%)	⬆️

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