Kerry earned all four: two on the Gridley and two on Swift Boats.
Check the campaign dates. He earned one for his Gridley cruise and the first part of his Swift time. They were both within one campaign period. A transition date between campaigns, Jan 29/30, 1968, fell during his time on the Swifts. The end of his Swift time was in that second campaign period.
He only qualified for two of them. He was wearing only two when he appeared on Meet the Press in 1971.