How do I make 4 measures to a system?

Adding breaks won't force MuseScore to squeeze more measures on a line than your current settings allow, it only forces it to put fewer than your settings allow. Just like a word processor - you can use
line break to
end lines early
but you can't put a whole bunch of words on a line and force them to all be on one line even though they fit because word wrap will still have its say.

If you want to squeeze more measures on a line than fit given your settings, you need new settings. Either smaller staves (Format / Page Settings / Staff space) or less space between notes (Format / Style / Measure, the Spacing and "Minimum note distance" setting. You can also select measures and press "{" one or more times to override the spacing for the selected measures, sometimes that's enough. All depends on the specifics of your score and whether you'd rather solve the problem by setting smaller notes, less space between all notes, or just less space in some places. etc.

That's because you have so many notes in the first three measures they need that space.

Try making everything smaller using Format>Page Settings and reducing the stave space. One click down should do it.