Changing 3 stanzas into 4 stanzas with music and lyrics already finished and in place.

Do you want to add a stanza (or verse) to the existing score? To add it below the existing ones, press ctrl+l.

If you want to add for example verse 2, you need to move verses 2 & 3 to the next verse. right click a word in the last verse, choose Select>More... and check the boxes for the same verse and same staff as appropriate. Click OK and you'll have all words in that verse selected. In the inspector, there is a field that says verse, increase that number by one and all of the words will move to the next verse. Literally, they'll move on the screen and make room for another verse. Work your way up until you have a space for the verse you want to add. Select the first word for the verse above where you want to add a verse and press enter to move to the next verse and start entering. If you stop in the middle of a verse you can select the last word in that verse and press space to move to the next note.

I wonder, though, if maybe you don't literally mean stanza here, but actually "system" - a line of music left to right across the page? If so, then simply add system breaks where you want the systems to end. Either using the Breaks & Spacers palette, or by clicking a measure and hitting Enter.