Teodorin Obiang Nguema, the son of the country's president, is accused of money laundering. He has not commented.
Swiss authorities have seized 11 cars in total.
Among them was reportedly a Porsche valued at more than $830,000 (£667,000) and a Bugatti Veyron which sells for $2m (£1.7m).
The accused is due in court next year in France on similar charges, which he denies.
Prosecutors in Geneva say he has plundered his country's oil wealth to buy luxuries, including a private jet and Michael Jackson memorabilia.