The key to the interest rate is the term, 30, 60, 90 days. In the cases of factoring I'm familiar with, not only is there the initial discount, but any money received begins bearing additional interest if not paid back in full in the agreed time frame.
As an example, business A agrees to factor $100,000.00 at 4% and receives $96,000.00. If they pay it back in 30 days, they pay $100,000.00, which is 4.1667%/mo or 50% APR. However, if they do not, the unpaid portion then bears an additional interest rate of 18% per annum until paid in full. I've seen small businesses, particularly home remodelers, buried by those terms.
"Libenter homines id quod volunt credunt"