The highest ranking military US member to actually be court-martialed was Major General Robert Walker Grow, who commanded the 6th Armored Division in WW2. He was serving as a senior military attaché in Moscow when excerpts of his personal diary were photographed and latter published in an inflammatory form by the Soviets. Grow welcomed a formal hearing because he expected it would clear his name, as none of the captured excerpts contained classified material, and wound up receiving an official reprimand and a suspension from command. Grow managed to get the sentence remitted in 1957 - the sensationalism that surrounded his court-martial had lessened significantly by then.

As for your second question, a court martial of a four-star general, while technically feasible according the Manual of Courts-Martial, is unlikely to occur today. Take the case of General William E. Ward, who was found to have perpetrated "multiple forms of misconduct" but was only demoted to (and allowed to retire) a major general.