The SJSU impaction index is HS_GPA * 800. Since the one for CS is 3400, dividing by 800 gives a 4.25 HS GPA needed, if the applicant does not have any bonus points (e.g. +200 (equivalent to +0.25 GPA) for being a local area applicant).
These were the competitively-determined admission thresholds for fall 2021 admission. Note that these numbers were published around the time they were issuing decisions for fall 2021 (not before), and that the page says that provisionally admitted applicants included “Applicants who met the impaction threshold for their primary major as listed below.”
While they have some predictive value for those applying for fall 2022, they can be different for fall 2022 due to changes in how competitive the pool is for each major.