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

[github-actions]

This pull request changes the compat entry for the Compat package from 3.15 to 3.15, 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 #44 (2a5be6f) into main (8c25fea) will decrease coverage by 0.39%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
- Coverage   96.20%   95.80%   -0.40%     
==========================================
  Files           5        5              
  Lines         158      143      -15     
==========================================
- Hits          152      137      -15     
  Misses          6        6              

Impacted Files	Coverage Δ
src/Checkpoints.jl	91.89% <0.00%> (-1.14%)	⬇️
src/indexing.jl	94.87% <0.00%> (-0.26%)	⬇️
src/handler.jl	96.77% <0.00%> (-0.20%)	⬇️
src/session.jl	100.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8c25fea...2a5be6f. Read the comment docs.