I want to create a score with 4 measures per line.....

The width of a measure entirely depends on it's contents.
You can prepare a score to have a system break every 4 measure (see https://musescore.org/en/handbook/3/layout-and-formatting#system-breaks on how to achieve that) it only guarantees you that you have at most 4 measures per system.
If you choose to then fill them with very short notes (or long lyrics or... whatever takes up space) and the result will make your measures to wide to fit all on a single system, MuseScore will still autobreak the system where required.

You can also not lock barlines so they don't move; as measure width depends on contents, so will the position of the barlines be adjusted to show those measure widths.

Now since your use case is easy lead sheets there are some configurations/tricks to explore:
* Set a larger minimum measure width
* Fill the measures with rests of the shortest duration in another voice, mark those invisible. They will still act as a layout grid

In case where MuseScore 4 is an option (currently in beta):
Set the Format > Style > Measure > Spacing Ratio to 2.
This'll make it so a note with double duration will also take up twice as much horizontal space as the other.