When you start VOCALOID4 Editor, it automatically generates one part with 32 measures. You can change the length of this part.

To change the length of the part, use the arrow tool (Right-click the Track Editor and then select "Arrow") to point to the rightmost end of the part (the area in light green) on the Track Editor. The mouse pointer changes to a red arrow.
Then, you can drag the pointer horizontally to adjust the length of the part.

If you want to enter notes in the 33rd measure or later, drag the pointer further to the right.