The IRS Form W-4 is required to be filled out every time there are changes in the amount of tax withheld from your paycheck. This form helps your employer calculate the correct amount of federal tax to be withheld from your pay. For example, when you are hired, your family situation changes, or you take out a loan with tax deductible interest, the math is different. Based on the information you provide via this form, your employer will calculate the take-home amount for your paycheck.
It is very likely that you will have to fill out the W-4 form multiple times when switching jobs or making tax withholding adjustments. So, it's better to fill it out once and store for repeated future use. You can add information to this form on any iPhone, iPad or Mac using the advanced productivity tool PDF Expert, the best PDF app to fill out tax forms.