I have a pop song that starts with four bars of piano before the vocal part comes in. How do I hide those empty vocal bars from the Full Score layout so the first line is just the piano part?
The traditional way is Layout Options > Vertical Spacing > Staff Visibility > Hide empty staves - set to All systems.
As of 3.5 you can add a System Break or Frame Break at the start of the flow, double-click it or hit Enter to open the Manual Staff Visibility dialog, then flick the switches accordingly. You’ll need another System Break with manual staff visibility set four bars later, to get your vocal stave back.
(Manual staff visibility breaks can also be added from Edit > Staff > Manual Staff Visibility or from the context menu.)
Apparently putting in chord symbols above the piano part brings back the vocal part bars 
…Good thing 3.5 gave me the alternative fix - that worked!