Bail bondsmen do not make up their fees. Fees are typically 10% with some variations depending on the state. There are no 4% bonds anywhere. Those advertised rates are usually a “down payment”.
If Bondsmen were permitted to charge whatever they wanted that would undermine the very job the judge is doing by setting bail in the first place. Bail is used as incentive for a defendant to appear in court, or incentive to a cosigner to guarantee the defendant appears in court.
The State and The Department of Insurance in each State is the licensing entity that determines how much Bondsmen charge for their fees.