I have Dorico for iPad. I have locked all systems at 4 measures, but I have one extra measure at the end that is all by itself on the page, so I need to add that measure to the 4 measures before it so there will be 5 measure in the last system. This is what A.I. told me to do:
Switch to Engrave Mode: Tap the mode selector in the top left corner of the screen and choose Engrave.
Select the Range: Select the first item (like a note or rest) in the bar where you want the system to begin, and then tap the last item in the bar where you want the system to end.
Apply the Command: Open the left-hand panel (if it isn’t already) and tap Make into System. [1, 2, 3, 4]
The problem is when I select the first note of the system I want, and then go to the last note, the first note unselects. So I can’t go to step 3.
Which suggests to me that your previous “…I have locked all systems at 4 measures…” was done manually rather than using Dorico’s layout options to set casting off to 4 bars per system.