If you choose the option to "erase all content and settings", you'll remain on 12.1.1. If you restore your device you'll get 12.1.4, and the oldest version of iOS you'd be able to manually install is 12.1.3.
If you choose the option to "erase all content and settings", you'll remain on 12.1.1. If you restore your device you'll get 12.1.4, and the oldest version of iOS you'd be able to manually install is 12.1.3.