Four major banking institutions with offices throughout the Washington region will establish an automatic teller machine (ATM) network that will provide service to customers in Maryland and Virginia, bringing interstate banking a step closer to reality.

And while participants in that shared system were preparing yesterday to disclose plans for the operation, a second group of regional banks, said to be much larger, was putting final touches on a proposal for a competitive network.

The first of these proposed interstate ATM networks will be shared by Suburban Trust Co. of Hyattsville, First National Bank of Maryland of Baltimore, First Virginia Banks Inc. of Falls Church and Savings Bank of Baltimore.

When the network becomes operational in early 1982, customers of the four participating banks will be able to use 173 automatic teller machines in either of the two states.

The new ATM network is actually the second interstate type for this region. The three First American Banks in the District, Maryland and Virginia unveiled their Cash Flow ATM network last year. However, those three institutions, although operating in separate jurisdictions, are affiliates of Financial General Bankshares Inc., a Washington-based holding company.

The latest network, like Cash Flow, will permit cash withdrawals from ATMs across state lines and enable customers to make transfers from one account to another. However, because of the ban against total interstate banking, customers won't be able to make deposits across state lines.

At least 250 machines will be operational by the end of the first year, officials of the banks said yestreday.

The shared network will be tied to Maryland Switch, an electronic funds transfer system based in Baltimore. Maryland Switch currently operates primarily as a utility for authorizing credit card transactions.

The shared ATM network "forecasts things to come," said Phillip A. Parker, president of First Service Co., a subsidiary of First Virginia Banks, a holding company based in Falls Church. "What we envision is that when the system becomes operational, small credit unions, small savings and loan associations and small savings banks that don't have ATMs or computers can provide their customers cards to participate in the network."

Representatives of the four banks were in D.C. yesterday for discussions with officials at other institutions who may be interested in joining the network. Parker said his group anticipates that the shared network will include additional institutions before it becomes operational.

Suburban Trust and First Virginia Banks had been active participants in discussions of plans to form a much larger shared network. However, when sharp differences caused a split among the participants, First Virginia and the other three banks decided to establish a separate ATM network.

"Part of the difficulty we had was a difference in philosophy," said Terry L. Eikenberry, vice president of Suburban Trust's future banking division.

"I think the philosophy of the group that started out envisioned a nationally prominent system that would make money for the group," Eikenberry said.

Parker and Eikenberry said that while their banks are interested in turning profits, it's equally important to develop economies of scale and to increase customer convenience. Moreover, they said, Maryland Switch is a "known quantity" that allows them to become operational quicker.

Representatives from another group of banks from the District, Maryland and Virginia are expected to announce plans next week about a new shared ATM network.

At least eight banks with offices "covering a much bigger piece of geography" are expected to form the new network, said a source close to the group.