How to convert 48V to 5V in this system?
The simplest choice would be any number of step-down modules designed for that voltage. 50V input to 5V output can be found for a few bucks (Pun totally intended), with more reliable resellers costing more.

In any case, keep in mind, a GSM shield, from what I could find, plus the arduino, requires 700mA up to 1 amp of power available. Size your regulator accordingly.

And size the input voltage to the max voltage, not just the nominal 48V. If your PV system is charging the batteries at 50+ volts, then your regulator needs to be able to handle that. A car for example, is 12V nominal, normally sees up to 14.5V while charging. In that range, a regulator that can handle up to 72V may be required.

Oh, and your Voltage Divider is incredibly wrong. 100k over 10k, to bring 48V to 5V with under a milliamp draw.

Update: If you are only pulling this power one time a day, for x minutes, then maybe you can power it from 2 batteries instead of all four. It's not recommended as a large draw can cause the batteries to discharge unevenly, but if its only a few minutes a day (I'm assuming long enough to gather data and send it via text) then that should be fine.