I agree with AJISH ALFRED here, you haven’t given us a lot to work with really.
If you require just 50% of the voltage, a simple on-off-on switch can be wired so that in one ‘ON’ position the battery is connected straight to the load and on the other ‘ON’ terminal, you could put a pair of resistors as a voltage divider to give you half the voltage.
BUT, in that simplest of configurations you would also be dumping half the power to the 0 volt line too, which is going to drain the battery just as fast at full power.
The best way to do this would be to use the LM338T voltage regulator which is capable of handling 5 amps and use the switch in the adjustment circuit. If you find the free ‘Texas’ datasheet from one of the many places online, there are several application notes you could calculate this from.
If you want current control, that same chip can achieve that function too. If you want to do both,.. yes that is also in the datasheet.
By using an adjustable voltage regulator, any switch can be fairly small as it is only handling low current. Switching 4 Amps will mean a much more expensive switch as the contacts will have to be a lot heavier. I think you may find a small switch and a handful of components may work out a cheaper option, as well as being more accurate, more adaptable and have a longer service life.
This circuit is from the datasheet and could be configured very easily to give you what you need.