You don't need a 4K TV to use the Xbox One X. That's a common misconception. While you get the most benefits from playing on a 4K HDR TV, you will see some improvements while playing on a 1080P TV.
The Xbox One X uses Super Sampling to deliver better looking gaming on a 1080P TV, so you get a benefit while playing today and are future proofed for when you upgrade to a new TV.
With Super Sampling the Xbox One X will help games look better on your HDTV by reducing "jaggies" around the edges of your screen and delivering other benefits like longer draw distances so you can see more of the world as you play. You will also see better special effects and overall a better looking game than if you connected your Xbox One or Xbox One S to the same TV.