bump compat poly v4

[baggepinnen]

Closes #898

[JuliaControlBot]

This is an automated message.
Plots were compared to references. 11/11 images have changed, see differences below.
After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here.

Difference	Reference Image	New Image
❌ 0.047
✔️ 0.011
✔️ 0.001
✔️ 0.001
✔️ 0.007
✔️ 0.003
✔️ 0.0
⚠️ 0.028
❌ 0.06
✔️ 0.0
⚠️ 0.017

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6e6a0d1) 34.06% compared to head (648c4d0) 34.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #899      +/-   ##
==========================================
+ Coverage   34.06%   34.16%   +0.10%     
==========================================
  Files          42       42              
  Lines        4624     4624              
==========================================
+ Hits         1575     1580       +5     
+ Misses       3049     3044       -5     

see 1 file with indirect coverage changes

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