The headline feature of the Insta360 Go 3S is undoubtedly the 4K video. I said in my review of the Go 3 that “I never once noticed the 2.7K footage from the Go 3 being 2.7K, which is to say, unless you shoot side-by-side with a [4K camera], most people would never be able to tell the image quality difference.” I stand by that. But when you are putting 2.7K side by side with 4K in the same video, you can tell the difference. For instance, I always found it difficult to mix footage from the Go 3 with footage from my GoPro, which I often shoot in 5.3K. This is where the real appeal of the new Go 3S lies. The 4K footage does objectively look better (though again, you need to put it side by side to see it). More importantly, it mixes naturally with 4K footage from other cameras like the GoPro Hero 12 (8/10, WIRED Recommends) or the Insta360 Ace Pro (8/10, WIRED Recommends).
What impressed me most about video from the Go 3S wasn't so much the additional sharpness, which is there, but the lack of pixelation, particularly in motion shots. Where the Go 3 tended to get quite pixelated in motion shots (e.g., while riding a bike), the Go 3S does not. This improvement is likely due more to the Go 3S's higher bit rate (120 Mbps versus 80 Mbps) than the 4K video, but either way it's a welcome improvement.
That said, there is still a good bit of pixelation at higher frame rates, so I recommend avoiding them. Shooting at 120 fps isn't too bad, but the 200 fps mode is often unusable. (The quality of the footage depends a lot on lighting, but even in pretty good light, 200 fps is too much for this sensor and lens to handle well).
The other very noticeable difference between video from the Go 3 and Go 3S is the improved dynamic range and color. The Go 3S has much better tonal range in shadows, showing a lot more detail. There was also none of the (slight) magenta color cast that I sometimes got with the Go 3.
Other Changes
Before you get too excited about smoother, sharper video, there are some other changes to the Go 3S to keep in mind. The first is that the sensor has changed. Previous Go cameras used an oversize sensor that allowed you to shoot either vertical or horizontal footage (using Freeframe mode) regardless of how the camera was oriented.
This changes in the 3S. From my testing and experimenting it seems that the new sensor is horizontal, and software is making up the difference. This results in slightly less clear vertical video than what you can get with the Go 3.
In fact, as someone who exclusively shoots horizontally for longer-form content, I would never have noticed this change were it not for legions of offended Reddit users endlessly arguing about it. Suffice to say that, If you're a huge fan of the Go 3's Freeframe mode, which you then crop to vertical video, the Go 3S might be a step backwards. I prefer to think this is Insta360's clever plan to encourage you to shoot horizontal video, as the gods intended.