If your ammonia is 4.0ppm you need to do a 75% water change to bring it down to 1.0ppm. A second 75% water change immediately after that will bring it down to 0.25ppm. That is where you want to try to keep it. If you have any doubt about that large of a water change causing issues I will be more than happy to do the same on any of my 20some running tanks, though the 150 gallon tubs will take a bit more time than immediate due to my water heater catching up. I have four 40 gallon tanks running on tap water ATM, close enough to 45, pick one & I'll have at it.
Fish do not get used to ammonia any more than you would get used to levels of carbon monoxide in the air you breathe. As far as changing charcoal, you don't need to run that unless you are removing medications from the water, use that space for more bio media.