There is no mention of the price of these shares. I can see two possibilities:

1. Everything above 2000 BTC is returned proportionately to the investors, so for example an investor putting up 20 BTC and there is 4000 BTC invested in total, he will get 20/2000=1% of the shares and 10BTC is refunded.
2. The price for each share is determined purely by demand, our investor with 20 BTC will get 20/4000=0.5% of the shares and no refund of BTC.

Which is it?