4% per year s not that attractive option anyway. Especially if you do not control your private keys. I d go with something where you can profit and still have your keys stored in an offline wallet.
How can they use those coins though?
They're offering investments to increase their portfolio. Some of the coins will be spent in this process. Otherwise it's free money, not and investment.
I believe "crazyivan" is talking about something else here. probably what he is advertising in his signature called DMD Diamond and if I understand it correctly it gives you staking reward for the amount you hold. it is similar to the profit you gain here.
I have to check the website for more details but it would have been great if he explained more himself.