Not really, a Mini Cooper has a payload capacity of 450kg according to some googling.
That gives us ~2.7 Minis.
If we factor in the weight of the driver, then we can comfortably get away with 4 Minis, each taking 312kg of gold which leaves us with 138kg per Mini for the driver and any gear that might be needed for the heist.
Your calculations are off by some margin. Assuming 312kg per car, it would take 3962 minis to transport it all, somewhat more than the 4 you accounted for. Of course you could do this with 4 cars over a thousand round trips - I'm not sure which option is most likely to lead to success, I suspect both might arouse suspicion.
