Except in the case of a taxpayer fraudulently or willfully attempting to evade the tax, any escape assessment shall be made and the taxpayer notified thereof within four years after August 1 of the year in which the property escaped assessment. No escape assessment shall be effective until the assessee has been notified personally or by United States mail at his address as contained in the official records of the board. Receipt by the assessee of a tax bill based on such assessment shall suffice as such notice.