How to make high voltage dc to low voltage dc ?
6v 4Ah to 6v 500ma.
When a circuit is supplied/powered by a certain voltage, it takes the current it needs only.
For example if you connect a resistor 1K (as a circuit) to a big 12V battery, the battery current will be 12/1K = 1.2mA only
The difference between a big and small 6V battery is that the former will last more in supplying the circuit current before it losts its charges ( charge = circuit current X time ).
4Ah means that , in theory, if the circuit current is 500mA, the time for the battery to become discharged is 4/0.5 = 8 hours.
As long the voltage is right for the circuit, there is no fear for the circuit to be burned.