All tax foreclosure actions begun prior to the effective date of this act shall be completed in conformity with the laws in effect prior to such date, and in all cases where tax sale certificates are held by persons other than the county, the redemption and conveyance of lands governed thereby shall be governed by the law in force prior to the effective date of this act. In all other cases the provisions of this act shall govern, but no suit for the foreclosure of tax liens on lands first bid off by the county at the tax sale held in September, 1940, or any prior year, shall be filed until the expiration of four years from the date of the first sale from which said land has not been redeemed.