Different share types
While Page and Brin do both have around four million Google shares, the founders retain the majority of the voting rights in the company after their sell. This has to do with the different types of shares Google issues.
Google has three types of shares: Type A, B and C. Type A has ten times less voting right than type B and type C has no voting right at all.
52% of voting rights
After the sale Page and Brin have 11.9 percent of the Google shares of type A and B left, but still have 52 percent of the voting rights in the company. The Google founders both sell 2 million shares of type C and 2 million of type B. Their shares of type B change after sale in Type A non-voting shares.
After the sale, Page and Brin have together about 40.6 million shares of series B and another 40.6 million Type C possession. Besides the founders mainly Google Chairman Eric Schmidt owns many type B shares.