Battery Bank - How to achieve full amp-hour ratings with simultaneous series & parallel connections?
Simple mathematics tells you that with 4 off 12v batteries, you cannot have 48v by connecting all 4 in series and more than 1 battery in parallel for 12v duty.

If you already have a use for 48v DC for the servo motors, and you still need your 120v AC, then I suggest you buy a 48v to 120v inverter. This is likely to be slightly more efficient than a 12v to 120v inverter.

A better solution would be to ditch the requirement for 120v, and build/buy something to run your steppers from 48v (using switching for constant current, not linear constant current, for better power consumption).

If you have some hi watt diodes, you could try selling them, and putting the money so raised in the kitty for the 48v inverter!