OS Details

Apple iPad Mini 4 has been launched in September 2015.
Apple iPad Mini 4 have iOS 9 , upgradable to iOS 10.3.3

Screen Dimensions

Apple iPad Mini 4 screen size is 7.9 inch with ~ 70.6% body ratio of actual device size. Physical dimensions for Apple iPad Mini 4 device is 8.0 x 5.31 x 0.24 INCH or ( 203.2 x 134.8 x 6.1 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 iPad Mini 4 resolution is 1536 x 2048 ^PX with ~ 324 pixel desnsity.
Apple iPad Mini 4 viewport size is 768 x 1024 ^PX with ~ 162 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 iPad Mini 4 have 7.9" physical screen size and its resolution is about 1536 x 2048 Pixels with approximately 324 PPI pixel density. Apple iPad Mini 4 has viewport size 768 x 1024 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 iPad Mini 4 Media Queries (In terms of Tablet only)

@media only screen and (min-width: 768px) and (max-width: 1024px) { /* Your Styles... */ }

Apple iPad Mini 4 Min-Width Media Queries

@media only screen and (min-width: 768px) { /* Your Styles... */ }

Apple iPad Mini 4 Min-Height Media Queries

@media only screen and (min-height: 1024px) { /* Your Styles... */ }

Apple iPad Mini 4 Landscape Media Queries

@media only screen and (min-width: 1024px) and (orientation: landscape) { /* Your Styles... */ }

Apple iPad Mini 4 Portrait Media Queries

@media only screen and (min-width: 768px) and (orientation: portrait) { /* Your Styles... */ }

Apple iPad Mini 4 Retina Media Queries

@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: 324dpi),
  only screen and (                min-resolution: 2dppx) { 
    /* Retina styles here */
}
                                

Retina is actually based upon device pixel ratio. Devices mostly have 2x or 3x display so you can use general retina media queries to display high resolution content on all kind of devices. Retina 2x and Retina 3x media queries given below:

Retina 2x Media Query

@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 */
}
                                

Retina 3x Media Query

@media
  only screen and (-webkit-min-device-pixel-ratio: 3),
  only screen and (   min--moz-device-pixel-ratio: 3),
  only screen and (     -o-min-device-pixel-ratio: 3/1),
  only screen and (        min-device-pixel-ratio: 3),
  only screen and (                min-resolution: 384dpi),
  only screen and (                min-resolution: 3dppx) { 
    /* Retina styles here */
}
                                

Apple iPad Mini 4 Specifications

• Launch Date September 2015
• iOS 9
• Physical Screen Size 7.9 INCH
• Resolution 1536 x 2048 PX
• Viewport Size 768 x 1024 PX
• Pixel Density 324 PPI
• Device-Independent Pixels 162 PPI
• Pixel Ratio 2 xhdpi
• Width 1536 PX
• Height 2048 PX
• Device Width 768 PX
• Device Height 1024 PX
• Dimensions 203.2 x 134.8 x 6.1 mm (8.0 x 5.31 x 0.24 in)
• Weight 299 g - 10.55 oz