Having the last row not print on the correct page is a common issue with labels layouts in FileMaker. Here's why it happens:
Almost any given plain paper printer's printer driver cannot print edge to edge from top to bottom of a letter size piece of paper that feeds through the page. A small area at the leading edge and a larger area on the trailing edge cannot be printed on. FileMaker compares the spaced needed, as specified by the layout design (Header height + Footer Height[if you have one] + Body Height) and computes the number of instances of the body that fit in the limits of that printable area. If 4 bodies + 1 footer + 1 Header exceeds that printable area by even 1 pixel, the 4th row of labels will be printed as the first row on the next page and you get an unused row of labels on your layout. Note that this printable area is not only different for each printer model, different printer drivers that print to the same printer model can differ slightly.
So you'll need to "tweak" your part heights in the Inspector slightly while using preview mode to check your results until you get the correct number of rows of labels on each printed page.
Thank you for the informative explanation. I started over, deleted the header and adjusted the parts in Layout mode, and it worked! Centering the fields in the label was a small challenge because the label width is measured from left edge to the left edge of the adjacent label, but I tweaked the text field a few times and I'm happy with the result.