How to keep bars with fixed length

Normally it would be considered improper engraving practice to keep fixed length for measure regardless of their duration. Measure width is supposed to vary not only with duration but also with content, so that for instance in 4/4 time, a measure with only a whole note should be narrowed than one with 16 sixteenth notes. And MuseScore obeys this standard practice.

However, although it is difficult to tell from just a picture - attaching your actual is much more helpful - I think what is going in your case is something different. I suspect you have a line break after the fourth measure, and that is the only reason those empty measures are so artificially wide. You can make the next four measures just as artificially wide by adding a line break there as well. You can use the Breaks & Spacers palette, or Edit / Tools / Add/Remove System Breaks. I am guessing you started with teh default empty score (which has line breaks already set every four bars) rather than creating your own score, and that is why you weren't aware there line breaks involved. And indeed, line breaks go away when changing time signature, because the locations of the barlines changes and so the original breaks may no longer make sense.