The max allowed output current of a little 2N3904 transistor is only 200mA and it performs poorly above 100mA.
Like most silicon transistors its reverse-biased emitter-base breaks down at 6V but most circuits never reverse-bias the junction, it is usually forward-biased.
A common-collector transistor has its input signal at its base and at ground, not at its base-emitter junction. Its emitter voltage will always be about 0.7V less than its base voltage for an NPN transistor. The 0.7V is forward-bias, not reverse-bias.
The transistor will get extremely hot with a current of 4A. Use a push-pull circuit instead like an audio amplifier IC.