There are three different versions of the Moto X4 sold in the United States. The Moto X4, the Moto X4 with hands-free Amazon Alexa, and the Moto X4 Android One. All have different software customizations, and all will work on Republic Wireless. The Android One version is sold exclusively through Project Fi, and may require activation on Project Fi to receive discount promotional pricing.
Seeing that you have had the Moto X for so long, you might be most comfortable with the standard Moto X4. This model will come with most of the Motorola features to which you have become accustomed. Motorola’s online store is selling it for $299 right now. If you decide to buy the phone from anyone other than Republic Wireless, you will need to check that you are in an area that supports “bring your own phone” and order a SIM card: