The rate is realistic. The only other rates we've seen have been 10% a day high-yield investment plan scams. A 4% interest rate is very sustainable.
As for how they'll use the money, they've provided very specific explanation on their website.
This is the explanation directly from their website:
When you hold money in your FreeBitco.in account, we split it into 3 parts - the first part is used to cover user withdrawals, the second part is used to bankroll our MULTIPLY BTC game and the third part is used to expand our bitcoin mining operation (which is currently over 6 Petahashes or 0.18% of the total hashpower of the bitcoin network). We make a profit and assume the full risks of our businesses and in return for you saving your money with us, we give you a risk-free fixed rate of return.
Below, we have signed a message using our bitcoin address that is used to collect payments from our bitcoin mining operation. This message can be verified on any bitcoin client or here to provide proof that we actually mine bitcoin.
I transferred some coin over to them to try out their offer. I've had zero trouble. They've paid me every day since deposit as promised. The payments are made at random times each day. They track your estimated daily, 7 day, and 30 day and 365 day interest payment. They track your interest earned. And they shows 7 days of history of interest payments.
They have a lot more to lose than they do to gain if they cheat us. Give it a shot!
the rate is quite reasonable, and they got pretty famous name for a faucet site, but I still not really 100% trusting my investing in the site, maybe when more people give their testimonial then I will consider for investing, for now I still doubt to invest in the site