The April 15 tax filing deadline is less than two weeks away, but taxpayers can get an extra four months to finish their returns by filing for an extension.

The Internal Revenue Service said taxpayers can seek an automatic extension through Aug. 17 by filing Form 4868. But they still have to estimate their tax bill and pay it by April 15, or face possible penalties and interest charges.Under the tax law, interest is added to any tax not paid by April 15; the current rate is 8 percent, compounded daily. And if the amount paid is less than 90 percent of the total 1997 taxes due, a late-payment penalty of 0.5 percent a month is assessed.

If taxpayers can't afford to pay the full tax bill, they can request an installment payment plan by filing Form 9465. The IRS will let taxpayers know within 30 days if it accepts the installment plan; there's a $43 set-up fee for the plans.

The extension and installment agreement forms, along with other IRS forms and instructions, can be downloaded from the Internet or obtained via fax machine. The IRS Internet site is: (http://www.irs.ustreas.gov). The IRS Tax Fax is: 703-368-9694.