It peaked at 4.05% in March. The last 2 months it went just below 4% as the Unknown category increased. For June the reverse happened, so 4.04% seems to be the real current share of Linux on Desktop as desktop clients were read properly/werent spoofed.
Statcounter numbers are to be taker with boulders of salt. you can look at many metrics and especially when you filter by country. you will see a lot of erratic unexplained changes. jumping down and then a few months lather up by sometimes up to double digits.