Straps released with the Series 3 and earlier models are compatible with the Series 4. The 40mm Series 4 band works with 38mm straps; the 44mm Series 4 band works with 42mm straps.
* Measure your wrist with a metric tape measure all the way around exactly where you wear your watch. You can download a printable tape measure here. Please be sure to measure in metric (mm) and read instructions to right about printing to size.
If you are between sizes, order the next size up. You can take these bands to any jeweler/watch repair and have a link or few links taken out for the best fit. Email email@example.com for questions regarding sizing.