4% year interest is relatively small. I also doubt the majority of the investor want to get coupon. On top of that, the buy-back is only happening after 10 years, so this investment looks very long-term. As mentioned by others, I think every investor should get different interest return, maybe minimum 4% while maximum is 12%, based on how much they contributed in the STO.
I posted this answer earlier but I think it's relevant to your comment as well
--->There are couple of things to keep in mind. The first is that the token holder receives quarterly AND yearly payouts. The quarterly fixed coupon equals 1%x4 = 4% per annum; while the yearly payout is determined by the performance of the loans. As an example, we could say that the yearly return is 6% + 4% from the fixed coupon = 10% per year over 10 years. Then think about how Bitbond will buy the tokens back at the original price and you have a very attractive proposition.
Also keep in mind that the BB1 token will be tradeable immediately thanks to the Stellar blockchain. This means that you can easily sell your BB1 tokens if you believe the bears are turning into bulls. For crypto investors who have lost money in 2018 because of the downturn, BB1 offers a kind of stable coin on steroids, where the price is pretty much fixed and you even receive payouts on a regular basis.