I'm glad you asked this! It's something I've rattled around in my head for a while, so hopefully I can help. As you said "laptop", I'll assume you want a laptop that you can play with on the go, but ...
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I'm glad you asked this! It's something I've rattled around in my head for a while, so hopefully I can help. As you said "laptop", I'll assume you want a laptop that you can play with on the go, but can also output 4K with a monitor in a sort of docked configuration. I'm assuming you don't want the laptop to play 4K on the laptop screen. From experience, playing on a screen smaller than 13" on the go is a bust, so I'm going to be looking at 13" and larger screens. Lastly, I'll assume that Stadia won't be the only thing you'll be doing on this laptop, so I'll look for modern specs. I personally play on a Pixelbook. It's small, lightweight, battery lasts forever, and used you can find them for ~$600 these days. It has a 16:10 aspect ratio so you'll get letterboxing, but that doesn't bother me. For a similar price you have the much newer Pixelbook Go. On the Windows side for the same ballpark price we have laptops such as the Asus Vivobook 15 for ~$400 with 8GB of RAM and a 10th Gen i3, backlit keyboard, and is fairly thin and light. For $449 we have the Lenovo IdeaPad 3 with a 14" screen, 8GB RAM, and a Ryzen 5 3500U. Going up from there we have a whole range of HP laptops starting around $500 with 15" screens, 8GB RAM, and a variety of processors that can all decode VP9 just fine and push 4K via HDMI. This is just to name a few suggestions. Really the sky is the limit depending on what spend range you have in mind. If a desktop is more your speed, the last PC I built on paper just for Stadia came out to around $400, or $500 if you wanted hardware DTS:X support.