The Northrop Grumman Corporation said yesterday that it planned to close four plants and cut 755 jobs. The aerospace and military equipment giant, which is based in Los Angeles, said the move was part of its strategy of streamlining operations to remain competitive. The closings and job cuts will result in a pretax charge of $90 million against earnings for the fourth quarter of 1996. Northrop said it would close plants in Great River, L.I., with 330 employees losing their jobs; Hawthorne, Calif., where 530 people will be laid off, and Stuart, Fla., where 250 jobs will be lost. More jobs could be lost at the company's Perry, Ga., plant, which will be closed in 18 to 24 months, but no decision has been made on the number of workers there who will be offered transfers. The company said it would be adding jobs at plants in Texas, Georgia, Illinois and Pennsylvania, thereby reducing the net job reductions. Northrop employs 46,600 people.