(a) Except in the case of a false or fraudulent return and except as otherwise expressly provided in this part, every notice of a proposed deficiency assessment shall be mailed to the taxpayer within four years after the return was filed. No deficiency shall be assessed or collected with respect to the year for which the return was filed unless the notice is mailed within the four-year period or the period otherwise provided. For purposes of this chapter, the term “return” means the return required to be filed by the taxpayer and does not include a return of any person from whom the taxpayer has received an item of income, gain, loss, deduction, or credit.