CMRA is custom made for all of the Apple Watch series.The band connection for the new Apple Watch Series 4 hasn't changed even though the screen size is larger. The 38mm CMRA will fit the 40mm Series 4 Apple Watch and the 42mm CMRA will fit the 44mm Series 4 Apple Watch. For all other Apple Watches pick the CMRA based on the size of the screen (either 38mm or 42mm).