Approximate dot values
At 203 DPI, 100mm is about 799 dots and 150mm is about 1199 dots. At 300 DPI, 100mm is about 1181 dots and 150mm is about 1772 dots.
A metric label is not exactly 4 × 6 inches
100 × 150 mm is approximately 3.937 × 5.906 inches; 4 × 6 inches is 101.6 × 152.4 mm. Select metric dimensions explicitly rather than assuming carrier labels are identical. At 8 dpmm the theoretical grid is 800 × 1200 dots; at 12 dpmm it is 1200 × 1800. Use proportional image fitting to avoid changing barcode aspect ratio. Check exported PDF paper size and a physical sample.