You can use a non-4K receiver with a 4K TV, but it's not recommended as it won't be able to deliver the best picture quality. A 4K receiver is necessary to get the best picture quality from a 4K TV. A 4K receiver can handle 4K signals and can pass them through to the TV without any loss in quality. Additionally, a 4K receiver can also support HDR (High Dynamic Range) and wider color gamut, which are essential for a true 4K viewing experience.
With a non-4K receiver, you won't be able to take full advantage of your 4K TV's capabilities. You may experience a loss in picture quality, and you won't be able to access all the features that 4K content has to offer.
In summary, while it's possible to use a non-4K receiver with a 4K TV, it's not recommended if you want the best picture quality and access to all the features that 4K content has to offer.