Maintaining DC voltage with more batteries
Do the simple. Since you are pretending to use 4 batteries, arrange them on two pairs and connect them on parallel. You will still get 3V, but with the double of capacity (so half the discharge time of your bank).

For your knowledge, I suggest, for this voltage drop (6V to 3V), to do a search for buck converters. Do not try linear voltage regulators, or you will have worst autonomy even with 4 batteries than the original arrangement (2 batteries).