A 37-year-old man was sentenced to life in prison yesterday after his fourth armed robbery conviction when a Prince George's Circuit Court judge invoked a rarely used 1975 multiple offender law at the request of county prosecutors.

Judge Howard Chasanow sentenced Leroy Macklin, of 1213 Benning Rd. in Coral Hills, under a Maryland mandatory sentencing provision that requires life imprisonment after three previous separate periods of incarceration for armed robbery, if the state requests the life sentence. Chasanow had no choice in imposing the punishment since prosecutors made the request under the law, said county Assistant State's Attorney Margaret Nemetz.

Prince George's State's Attorney Arthur A. Marshall Jr. said the provision has been used in the county no more than 10 times in the last few years.

In a separate case, David Rhodes Randolph, 31, of 3417 Allison St. in Brentwood, pleaded guilty last week to the Jan. 9 second-degree rape of a 9-year-old girl. In pretrial conferences, Prince George's Circuit Court Judge Audrey Melbourne agreed to impose a suspended 20-year sentence and 5 years of strict probation in the plea-bargain arrangement, Marshall said.

Melbourne imposed the sentence to allow Randolph's rehabilitation, said Randolph's lawyer, Leonard Casalino. He also said the judge wanted to spare the child the ordeal of trial, which was set to begin today. Sentencing is set for Nov. 12.