lithium ion battery in series discharging question
The batteries in series will discharge uniformly. In first approximation that is. Due to cell differences the battery with the lowest capacity will define when the battery is empty - there will be charge left in the other cells but if you use it, you will overdischarge the other cell and damage it. 

To get around this limitation you would need an active cell balancing circuit which will put the charge from the higher cells onto the lower cells making it possible to use all the charge there is.



But you won't get a stable voltage out of it. The 12 V battery is internally made out of series connection of single cells. A single cell provides a nominal 3.6 or 3.7&nbsp;V. So it is likely either a 3 series or 4 series connection inside your batteries.

If you fully charge a lithium ion cell it'll reach 4.2&nbsp;V. If it is fully discharged it will be at 3&nbsp;V. So your 12&nbsp;V battery will vary from  16.8&nbsp;V down to 12&nbsp;V for a 4 series construction or from 12.6&nbsp;V down to 9&nbsp;V for a 3 series construction.

So if you put those in series again, you will get a nominal 48&nbsp;V but it will vary even more. If you need a stable output voltage, you have to put a regulator behind it. If your needed voltage is always lower than the lowest battery voltage, you can use a buck converter, otherwise a buck-boost approach would be recommendable. Or always boost when the battery voltage is always lower - but buck converters usually provide a bit better efficiency.