Having trouble getting your act together in time to meet the April 17 income-tax deadline?

Consider joining the 6 million or so of your fellow citizens who are turning for help to Form 4868 (Application for Automatic Extension of Time to File U.S. Indiviudual Income Tax Return).Complete this form properly, mail it to the Internal Revenue Service, and you'll get four more months, until Aug. 15, to complete your annual accounting with Uncle Sam.

This year, for the first time, the IRS will even send you a computer-printed notice acknowledging that it has received the form and granted your request. Previously, it notified only the relatively small number of people whose requests were turned down for some reason.

There's one catch, however. Extension of time to file does not mean extension of time to pay.

With Form 4868, the rules stipulate, you need to enclose a check big enough to ensure that at least 90 percent of your eventual tax bill for 1988 is paid.

If you don't meet that standard, you can run into a wide variety of woes, including penalties for late payment and a possible ruling that invalidates your extension request altogether.

So if you're uncertain how much your tax is likely to run, it makes sense when estimating it to err on the high side.

The trouble is, that means you're probably going to wind up giving the government the free use of some of your money for at least several months.

Drawbacks notwithstanding, the IRS says 5.7 million taxpayers filed for extensions in the tax filing season last year. For this year, it is projecting that 6.2 million will do so.

There used to be another way to duck the deadline and to postpone the need to pay as well.

All you had to do was be out of the country, defined as the 50 states and Puerto Rico, on April 15. That qualified you for an extra two months to file and pay.

"This was the easiest extension to get and particularly useful to some taxpayers who were not in a position to determine what their tax liability was going to be," said Steven Holub, managing partner in the Houston office of the accounting firm of Laventhol & Horwath.

About a year ago, a clever thinker in an airline's marketing department hit on the idea of promoting its flights to other countries as a way of getting this break.

But the idea was too clever in the eyes of the IRS, which immediately began a crackdown. Now the only people eligible for the extension are those who live outside the United States, including members of the armed forces stationed in other lands.

For people who will be traveling abroad this year, he adds, it is now necessary either to get your return in on time or file a Form 4868 before you depart.