OS Details

Apple Watch Series 4 (40mm) has been launched in September 2018.
Apple Watch Series 4 (40mm) have watchOS 5.0

Screen Dimensions

Apple Watch Series 4 (40mm) screen size is 1.57 inch with ~ 57.0% body ratio of actual device size. Physical dimensions for Apple Watch Series 4 (40mm) device is 1.57 x 1.34 x 0.42 INCH or ( 40 x 34 x 10.7 MM).

Displaying of Units are in this order "Height x Width x Thickness"

Screen Pixel Density and CSS Pixel Ratio

"Pixel" is smallest unit/element of any display.

The total number of pixels that fit into an inch is referred to as "Screen Density" or "Pixel Density" and it measured as "Pixels Per Inch".
Pixel Desnsity have limits to display Pixels Per Inch depends on different screen sizes. When number of pixels per inch increases than it's screen-size limit, the Display resolution will increase but actual device width/height in pixels remain same. Actual device pixels are known as Device Independent Pixels or CSS Pixel Ratio.

Apple Watch Series 4 (40mm) resolution is 394 x 324 ^PX with ~ 326 pixel desnsity.
Apple Watch Series 4 (40mm) viewport size is 197 x 162 ^PX with ~ 163 actual pixel density, which means it have 2 xhdpi display pixel density.

Screen Resolution and Viewport

The sum of pixels which are displaying on a device is called as "Screen Resolution". and sum of actual pixels of any device is reffered to as "Viewport".

Apple Watch Series 4 (40mm) have 1.57" physical screen size and its resolution is about 394 x 324 Pixels with approximately 326 PPI pixel density. Apple Watch Series 4 (40mm) has viewport size 197 x 162 Pixels and its pixel ratio is about 2.

For better understanding screen actual sizes, viewport sizes, display resolution and about their density click here.

CSS Media Queries

Apple Watch Series 4 (40mm) Media Queries

@media only screen and (min-width: 197px) and (max-width: 319px) { /* Your Styles... */ }

Apple Watch Series 4 (40mm) Min-Width Media Queries

@media only screen and (min-width: 197px) { /* Your Styles... */ }

Apple Watch Series 4 (40mm) Min-Height Media Queries

@media only screen and (min-height: 162px) { /* Your Styles... */ }

Apple Watch Series 4 (40mm) Min-Width Media Queries

@media only screen and (max-width: 197px) { /* Your Styles... */ }

General Media Queries for Smart Watches

@media only screen and (max-width: 319px) { /* Your Styles... */ }

In current era smart watches have no viewport size greater than 319px. So 319px is choose as max-width, also it will not disturb your styles for any kind of mobiles.

Retina 2x Media Query for Smart Watches

@media
  only screen and (-webkit-min-device-pixel-ratio: 2),
  only screen and (   min--moz-device-pixel-ratio: 2),
  only screen and (     -o-min-device-pixel-ratio: 2/1),
  only screen and (        min-device-pixel-ratio: 2),
  only screen and (                min-resolution: 192dpi),
  only screen and (                min-resolution: 2dppx) { 
    /* Retina styles here */
}
                                

Apple Watch Series 4 (40mm) Specifications

• Launch Date September 2018
• watchOS 5.0
• Physical Screen Size 1.57 INCH
• Resolution 394 x 324 PX
• Viewport Size 197 x 162 PX
• Pixel Density 326 PPI
• Device-Independent Pixels 163 PPI
• Pixel Ratio 2 xhdpi
• Width 394 PX
• Height 324 PX
• Device Width 197 PX
• Device Height 162 PX
• Dimensions 40 x 34 x 10.7 mm (1.57 x 1.34 x 0.42 in)
• Weight 40 g - 1.41 oz