Sen. John Glenn (D-Ohio) said yesterday he will introduce legislation freezing postal rates for private individuals at the current 15 cents per letter for four years.

Glenn, chairman of a subcommittee that oversees the Postal Service, said his measure "would benefit consumers - most notably those on fixed incomes - and help maintain a higher volume of mail, which is needed to sustain the huge system for six-day-a-week delivery."

A 15-cent rate went into effect last month, and Postmaster General William F. Bolger has said another increase would be needed in about 2 1/2 years.

Under Glenn's bill, Congress would authorize funds to cover the difference between the 15-cent rate for individuals and any higher rate imposed during the four years.

In House-passed legislation, which President Carter has refused to support, the subsidy would be increased immediately, and political control over the Postal Service would be increased by letting the president appoint the postmaster general.

Glenn said the administration has not explicitly endorsed his bill either, but said, "The White House supports almost all of this bill." He termed it a transitional measure while Congress studies what mail services the public wants and how to finance them.