Look at your portable charger's battery power capability compared to cell phone battery. If for emergency situation then it can make sense. A SEPIC converter is usually less efficiency in power conversion then a straight buck converter.
Take four alkaline batteries. They are about 2000 mAH's drawn down to 0.9 vdc. Let's say average cell voltage is 1.45 vdc with 2000 mAH's. Time four, that is 5.8 vdc at 2000 mA hours, 11.6 WH's. A smart phone battery at 3.7vdc average voltage at 1500 mAH is 5.5 WH's. Your power converter will have loss. The cell phone translation from 5 vdc input to 4.2 vdc peak voltage has loss. The set of alkaline batteries will provide about 1.8 full recharges, a rather expensive recharge cost.