If I make a 48v system with 4x 100ah battle born and use and inverter that outputs 12v for normal appliances. Will the 100ah last any longer due to the fact im only using 12v out of it?
Just to be clear, a device that converts from 48V DC to 12V DC isn't an inverter. It can be called a DC converter, a voltage regulator, a buck regulator, or a few other things, but not an inverter. An inverter turns DC into AC.
When thinking about a variety of voltages also think in terms of watts. If you have an appliance that uses 120W, then it's going to need at least120W from your batteries. It does not matter what voltage your battery is or your appliance is. 120W is 120W. You can't power a 120W appliance by using less than 120W from the battery. In this case:
120W to 12V appliance = 10A at 12VDC
120W from 48V battery = 2.5A at 48VDC
The current an volts are different, but the watts are the same.
You'll actually use a little more than 120W from the batteries because of losses in the connections, wires, and conversion.