Form DE 4 determines how much California Personal Income tax is withheld from your paycheck. This amount usually shows up on your paystub labeled "California State Income Tax". The amount withheld is based on your income, how often you are paid, your filing status, the number of dependents you have, and your expected itemized deductions.
Note that California State Income Tax is far from the only tax that is withheld from your paycheck. There's also federal tax, FICA/Medicare, California SDI, etc. Those each have their own withholding formulas.