How much current flows through a circuit if the if the resistance is 4 ohms and there is a 12-volt battery?

When two 4-ohm resistors are connected in parallel across 12 volts:

-- Each resistor has 12V across it, so the current is I = E/R = 12/4 = 3 amperes.

-- The total current from the battery is 6 amperes.

-- The effective resistance as seen by the battery is R = E/I = 12/6 = 2 ohms.

-- If you didn't know the voltage of the battery, you would calculate the

effective resistance of the parallel resistors in advance like this:

1/R = 1/4 + 1/4 = 1/2

R = 2 ohms.

When you connected them to a battery, the battery would agree.