how to use app store by 4G not WIFI

Make sure Cellular Data is turned on, and the switch for the App store App is turned on in Settings->Cellular->"Use Cellular Data For"->App Store.

Then you can use 4G to download Apps as long as they are less than 100MB in size.

If they are larger than that, you won't be able to download them over Cellular Data, only wifi.

Using Cellular Data on your iPhone - Apple Support