Looking side-by-side, the differences are minimal, almost imperceptible. The white version has a slightly different color scheme. From a top-down view, this area on the 4th generation protrudes slightly more than the 3S, with only a very subtle difference in feel. The touchpad (the area with haptic feedback) on the new model has a rubber-like texture. I'm currently using the so-called Mac version, which has a slightly different color scheme like Space Gray. In reality, the only difference between the Mac version and the regular version is the packaging — it doesn't include a USB receiver. I already have one, so it's compatible. I'm currently using the one that came with the 3S. So the only difference is the unboxing accessories. The Mac version focuses on Bluetooth connectivity.
White is 3S, black is 4.
To be honest, in terms of performance, I can't feel any difference in Bluetooth or the receiver. I don't use it for intensive gaming; it's primarily a productivity mouse. I'm currently connected to both this PC and my MacBook Pro, and I can seamlessly switch between them using the bottom button; the indicator light shows the currently connected device.
Weight and Build
In terms of weight, the new model does feel slightly more "solid," but the difference is very subtle unless you compare them repeatedly. It feels slightly larger in hand. The buttons are wider, and the overall structure is also wider. The buttons are more fully integrated, and the surface is more seamless, while the 3S has a slight separation between the scroll wheel and the base.
Scroll Wheel
The scroll wheel has always felt great. The official description says the 4th generation has improved it, but honestly, both generations have excellent scroll wheel feel. If you liked the 3S's scroll wheel feel, the new model won't disappoint.
There are also changes on the side. The 4th generation has a slightly more concave side, while the other side has a convex beveled edge design, making the side scroll wheel look more integrated into the body rather than protruding. Functionally, I didn't notice any significant advantage or disadvantage. The side scroll wheel itself seems a bit longer, with a larger thumb contact area. However, I don't use it that much; if you use it frequently, you might notice the difference more. The scrolling resistance is still very comfortable.
I quite like the small raised design of the thumb rest. Switching between the thumbwheel and this area is smoother, unlike the slightly recessed feel of the 3S. These are subtle differences, but if you're watching comparison videos, you're probably looking for these details.
Buttons
Regarding buttons, the 4th generation has an extra button. I haven't used it much yet. I use the original forward and back buttons extensively and customize them. I haven't found a suitable use for the new third button. I tried setting it up once while recording a video, but couldn't program it the way I wanted, so I ended up using the scroll wheel instead.
The top button shape has also changed; it's larger and no longer square, but designed along the diagonal lines of the body, making it more harmonious overall.
Haptic Touch Area
Next is the new haptic feedback touch area. Pressing it brings up an overlay menu on the screen, allowing you to set various shortcuts. Honestly, I hardly use it in my workflow. I often accidentally press it, and having to close the menu after it pops up is quite annoying. I prefer using keyboard shortcuts; they're more efficient. If I need customized operations, I'd rather have them tied to a physical button than press it and then select from a small interface. This might be more intuitive for those using a stylus or graphics tablet. But for a mouse, I feel it's more of a gimmick.
However, I quite like the feel of this entire area; it's more rubbery than the 3S. My white 3S has noticeably discolored after years of use. The 3S has a rubber coating that wears and yellows over time. The 4th generation's material is a combination of hard plastic and rubber, which I personally prefer.
Durability and Everyday Use
For everyday use, the 3S is actually very durable. I even considered buying another 3S, but I wanted to try the 4th generation. I'm very satisfied with the experience. It feels a bit more substantial in my hand, and I quite like that.
If you choose any of them, you won't be disappointed, especially if you're already used to the feel of the MX series. If you like the 3S, continuing to use it is perfectly fine.
Software
Speaking of software, this is where many people criticize Logitech. To fully utilize its features, Logitech Options Plus must run in the background. This allows for customization of keyboard shortcuts, application-specific bindings, mouse sensitivity, and more, offering more granular settings than the system's default settings. Some people dislike having software running in the background. I personally don't mind. It would be even better if settings could be directly written to the device, but that's not currently possible. However, on modern PCs or Macs, its resource consumption is almost imperceptible. I consider it a minor annoyance rather than a serious problem.
Mouse Testing
Then I tested it with an online mouse testing tool mousetester.net—it's very sensitive and performs very well.
Verdict
Overall, I like both mice. If the 3S hadn't started developing some minor issues, I would probably have continued using it. I'm glad I tried the 4th generation and plan to use it for a long time. If its durability is as good as the 3S, I expect it to last for many years.