If it has gone from 3% to 4% that would mean there are 33% more linux users and just don't see that as being true. That's the kind of increase you would definitely notice if you're the kind of person who hangs around linux haunts on the internet.
To be fair my suspicious is steam deck users likely make up a larger chunk of recent desktop Linux growth and aren't (as) likely to go to typical linux spaces online. Though since this is based on browser data, I also wonder how many steam deck users are actually browsing the web on them, so perhaps that my be a bad assumption on my part