i cant use 4G my ipad mini

Hello there, pkfromepune.

It sounds like you're not connecting to your carrier's 4G network with your iPad mini. The following Knowledge Base article offers up some great troubleshooting for cellular network connectivity:

If you can't connect to a cellular network or cellular data - Apple Support

Although the entire article has great information, this section might be most applicable in your instance:

If you can connect to 3G but not 4G LTE

If you can connect your iPhone or iPad to 3G but not 4G LTE, make sure your device is compatible with LTE. Go to Settings > Cellular > Enable LTE. Identify your iPhone and compare its model number with the list of supported LTE carriers for your country location. Or identify your iPad and compare its model number with the list of supported LTE carriers for your country location. LTE options might vary depending on your carrier's network.

Thanks for reaching out to Apple Support Communities.

Cheers.