President Reagan elevated national security adviser Lt. Gen. Colin Powell to the rank of four-star general Thursday and appointed him head of all Army ground forces in the continental United States. White House spokesman Marlin Fitzwater said Powell, 51, will assume in April his new command as head of the Forces Command at Fort McPherson, Ga., the largest military organization in the Defense Department, involving 1 million members of the Army. Powell had been in charge of all U.S. Army troops in Europe before going to the White House. The promotion was expected to put him in line to be chairman of the Joint Chiefs of Staff in about a year. The Jamaican-born Powell will succeed Gen. Joseph Palastra, who has retired. Powell will be in charge of preparing U.S. troops for worldwide deployment; the defense of the continental United States; and combat training, Fitzwater said.