Analysts polled by Thomson Reuters predicted a 4.8 percent rise in the figure.
Sales at stores open at least a year is a key gauge of a retailer's health. It excludes results from stores recently opened or closed.
Costco Wholesale Corp. said Thursday that the figure also increased 6 percent in the U.S. Internationally, the metric climbed 4 percent.
Excluding higher gas prices and the fluctuation of foreign currency exchange rates, sales at stores open at least a year rose 7 percent abroad.
Total revenue for the four weeks ended June 1 rose 8 percent to $8.78 billion.
For the year to date, the wholesale club operator said that sales at stores open at least a year climbed 4 percent. In the U.S., the figure also increased 4 percent. It rose 2 percent overseas.
Stripping out higher gas prices and the fluctuation of foreign currency exchange rates, sales at stores open at least a year rose 6 percent. The metric climbed 4 percent for the U.S. and 7 percent internationally.
Total revenue climbed 6 percent to $81.99 billion.
Costco, based in Issaquah, Washington, runs 657 warehouses, including 464 in the U.S. and Puerto Rico, 87 in Canada, 33 in Mexico, 25 in the U.K., 20 in Japan, 11 in Korea, 10 in Taiwan, six in Australia and one in Spain. The company plans to open up to an additional six new warehouses before the end of its fiscal year on Aug. 31.
Last month Costco reported that its third-quarter profit climbed as sales rose and membership fees improved. Its revenue topped analysts' estimates, but earnings fell short of Wall Street's view.
Its shares finished at $116.31 on Wednesday. They are down almost 2.3 percent since the start of the year.