windows 7 und musescore 4

There is more to it than the OS. You also need some pretty hefty computer specifications if you want to use muse sounds. You may not need MuseSounds to do what you want.

Otherwise,if you install a free program called 7-Zip, you can extract a mscx file from a MU4 file that will open in MU3

You wrote:
I'd like to edit a score saved with V4.
(...ich möchte gern aber eine mit V4 abgespeicherte Partitur bearbeiten...)

Okay, say you have a MuseScore 4 file named "String Quartet.mscz" which you wish to open in MuseScore 3. The ".mscz" file extension denotes a zipped file. Use a software like 7-Zip to unzip. (Depending on which software you use, you may need to re-name the ".mscz" file extension to ".zip" prior to extraction.)
In the newly uncompressed folder look for "String Quartet.mscx" -- notice the .mscx file extension. Open this "String Quartet.mscx" in MuseScore 3. MuseScore 3 may warn about the score being from a newer version but disregard.
[After opening the score in MU3, some formatting (e.g., text in the title frame) may need adjustment.]

You wrote:
Is there a solution in the meantime or am I now locked out? Then, of course, I will cancel my membership immediately.
(Gibt es inzwischen eine Lösung oder bin ich jetzt ausgesperrt? Dann werde ich natürlich meine Mitgliedschaft sofort kündigen.)

You are currently at musescore.org which supports the free MuseScore scorewriter software available here:
https://musescore.org/en/download

There is a difference between dotorg and dotcom:
https://help.musescore.com/hc/en-us/articles/210257565-What-is-MuseScore
and:
https://musescore.org/en/node/277874
The money you spent was for a Pro subscription at dotcom:
https://help.musescore.com/hc/en-us/articles/12532645611410-Why-does-Mu…-
and:
https://musescore.com/

A subscription to dotcom is not required to use the free MuseScore music notation software.