Civ. Code, § 2911

A lien is extinguished by the lapse of time within which, under the provisions of the Code of Civil Procedure, either:

1. An action can be brought upon the principal obligation, or

2. A treasurer, street superintendent or other public official may sell any real property to satisfy a public improvement assessment or any bond issued to represent such assessment and which assessment is secured by a lien upon said real property; whichever is later.

Anything to the contrary notwithstanding, any lien heretofore existing or which may hereafter exist upon real property to secure the payment of a public improvement assessment shall be presumed to have been extinguished at the expiration of four years after the due date of such assessment or the last installment thereof, or four years after the date the lien attaches, or on January 1, 1947, whichever is later, or in the event bonds were or shall be issued to represent such assessment, the lien shall then be presumed to have been extinguished at the expiration of four years after the due date of said bonds or of the last installment thereof or of the last principal coupon attached thereto, or on January 1, 1947, whichever is later. The presumptions mentioned in this paragraph shall be conclusive in favor of a bona fide purchaser for value of said property after such dates.