Where can you borrow money at 4% without collateral? Or even 5%? I know of a few stock exchange listed companies that would like know your secret. In fact, it seems to me that you could make a bit of money by helping to connect lenders who are willing to make multi-decade loans at 4% and students who want loans at less than 6.8% (more or less the current private lender student loan rate).
4% with collateral ~ actually you can get mortgages for less but given the amount of equity you need to have the actual rate (since you won't be earning interest on that equity) is nearer 4%.