Adding 4 batteries and making it 6v should work for 2500mA for 4 hours. So basically it depends on the load.. rite??
Not quite ... You are using the term battery to mean 2 different things.
A single "battery" is properly a CELL. Several cells in series of parallel is a BATTERY. Comes from the idea of artilery. 1 gun is just a gun, a group of guns is a battery.
Cells in series add their voltages so 4 cells in series for 6V is still only 2500mA for 1 hour. 4 cells in parallel would add their current capacity and provide you with 1.5V at 2500mA for 4 hours.