Case size

The Series 4  Watch come in two sizes (40 and 44 mm). All bands that fit the previous 38 mm models will fit the newer 40 mm models (same for 42 and 44 mm models). the larger watch face allows for a much better watch experience. You will hardly notice the extra two mm (.08 inch).

If you have a case or cover to protect your 38 mm  Watch it probably will not fit the newer 40 mm size. Contact the manufacturer of the case to find out.

Dear Apsm01,

Thanks for posting your question here at the Apple Communities. According to the checkout page, the Apple Watch Series 4 comes in two sizes:

1. 40mm
2. 44mm

I hope this helps!