Is it possible to use 4 HomePod minis to an Apple TV in one room and if not why not

Apparently not. One or two HomePods only:

See Set up home theater audio with HomePod mini or HomePod and Apple TV 4K

Here's what you need

• Apple TV 4K
• One or two HomePod mini* or HomePod speakers. Other AirPlay 2-compatible speakers are not supported with home theater audio.

* HomePod mini doesn't support Dolby Atmos 5.1 or 7.1 surround sound. 

why hasn’t this happened yet and is it coming

We will find out if Apple chooses to release any information at a later date.

suppose you can suggest it to them

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