Cheaper OPEC oil may be just down the road.

There is a definite possibility that the official price of OPEC oil could drop by $4 a barrel after a meeting of OPEC ministers in Vienna Dec. 19, say sources close to Arab officials.

If it happens, consumers might not see much lower gasoline or home-heating prices. Instead, many governments, including the United States, are expected to consider imposing a new tax on imported oil - to keep prices level, dampening consumer demand for oil and raising government revenues.

Even if the official price of OPEC (Organization of Petroleum Exporting Countries) drops, Iran, Libya, and Venezuela may keep on selling oil at a discount as they are now and ignore OPEC quotas, perhaps triggering a price war that could drive prices down even further.

Echoing widespread concern about a possible price drop, the chairman of British Petroleum, Peter Walters, said Thursday that any reduction in Vienna could set off a downward spiral that would endanger the world financial system.

If OPEC nations lower their prices, the argument goes, they have less money to put into international banks, and the banks would have less money to lend to developing countries with high debts. If just one developing nation defaults on a loan, confidence in international banking circles could be seriously eroded. The world would face a ''liquidity crunch.''

At the same time, developing countries that are oil-producing nations, either in or outside of OPEC, would also find their oil assets worth much less and face a financial crisis.

Sources close to Arab officials in the Gulf say Gulf states threatened a drop on the order of $4 in meetings here and in the Mideast on the eve of the OPEC ministers meeting in Vienna Dec. 19.

The hope of Saudi Arabia was that the threat itself was enough to bring OPEC ''rogue elephant'' members such as Iran, Libya, and Venezuela back into line with the current official price of $34 a barrel.

All three ''rogues'' need cash urgently in a buyers market. All are producing well above OPEC production quotas set last March, and are selling at substantial discounts in a variety of ways.

The threat may work, sources predict, but most feel that Vienna will see more disarray in OPEC, not less, and will end without definite price decisions. Discounting will continue as world demand remains low.

OPEC, the argument goes, faces the biggest-ever threat to its existence. And a $4 drop, taking the basic price for Arabian light crude oil down to $30 a barrel, would be Saudi Arabia's way of trying to reimpose OPEC unity and cohesion.

The decision could be taken at the next OPEC meeting, or at a session of the Gulf Cooperation Council (GCC) soon after the Vienna session. The council is made up of Saudi Arabia, Kuwait, the United Arab Emirates, Oman, Bahrain, and Qatar.

Saudi Arabia's oil minister, Ahmad Zaki Yamani, supported by Kuwait and other moderate Gulf states, is reported to prefer to protect the $34 price if possible , but to be ready to cut at least $4, if necessary, to make OPEC oil more attractive.

Mr. Yamani is also being pressed to lower the Saudi base price by the four major oil companies in Aramco, which are losing money by paying Saudi Arabia $34 a barrel when spot prices are already as low as $26.

''The one way OPEC can regain more of the world market share for oil is to sell more cheaply,'' says a veteran industry source just back in London after visiting the Gulf.

''OPEC simply can't keep the $34 mark when demand is so flat. It's going to stay flat next year. Yamani agreed to go up from $32 to $34 earlier this year to keep OPEC unity. Well, there's no unity now.''

An experienced nonindustry observer of the oil scene for many years agrees. ''Four dollars, I think,'' he predicted after talks with senior Arab oil officials.

The conventional wisdom is that there is a world oil glut, and that it is expected to continue until 1984 or later. A dissenting view comes from the 21 -nation consumer watchdog in Paris called the International Energy Agency, which argues that the glut is an illusion caused by a rundown of stocks. By the end of this decade, the IEA says, oil shortages and higher prices will recur.

Industry sources reply by pointing to new oil indications off California and China. They say oil reserves will be abundant for many years to come. They agree that world demand will pick up when recession eases, but make two further points:

* Demand will rise slowly because many people have insulated their homes and bought small cars, and many computers are in place, using less energy than big factories.

* If OPEC survives, or is replaced by a Gulf-state group, it will not repeat the experience of the 1970s and let prices jump too fast.

Today, prices have come down on spot markets to as low as $26 to $28 a barrel for Arabian crude. OPEC rebels are trying to cash in, and non-OPEC states, such as Britain and Norway, are being forced to keep their own prices low to compete.

A number of sources agree that a $4 OPEC drop would simply force Libya, Iran, and Venezuela to lower their own discounted oil to as low as $22 to $24 a barrel.

''If we then had a free falling price and a price war,'' comments one source, ''all kinds of implications follow, none of them good.''

There is a degree of unity on what those implications may be. From sources as diverse as pro-OPEC Mideast specialist Robert Mabro of St. Antony's College, Oxford, and Michael Roeskau of the IEA in Paris come this listing:

* The world banking system would be faced with less OPEC money to lend to debt-ridden developing nations.

* Consumer nations would be tempted even more to relax efforts to conserve energy, concentrate on coal, and develop synfuels and nuclear energy. Falling prices already make many synfuels projects uneconomic.

* New financial difficulties would lie ahead for other non-OPEC states such as Britain and Norway, whose oil revenues would drop at a time of deep recession.

Oxford's Mr. Mabro and other sources see Vienna as paving the way for yet another OPEC meeting next year, which could then see a price cut.

According to another scenario, chaos in Vienna will be followed within a week by a meeting of the GCC. That meeting would decide to lower the standard price to $30.

The IEA sees world consumption back up to 50 million barrels per day or 56 bpd if economic growth is higher. Current world consumption is around 45 million bpd.