is there a simple way to
- make all bars the same width?
- have only and exactly 4 bars per line?
You can insert line breaks anywhere you like - see Breaks and Spacer in the handbook. There is also a plugin that will automatically insert breaks every X bars; it is preinstalled and can be found under the Plugins menu.
No simple way I know to make measures the same width, although I suspect there might be away of getting close to that effect if you played hard enough with the General Style settings.
Menu -> Style -> Edit General Style... -> System -> Fix Measure Width
A lot easier than I thought :-) Nice, thanks!