Just picked up an S4 a couple weeks ago. Subjectively, the standby endurance of the OEM battery is relatively good if you turn off GPS, but drops rapidly when the screen is running. Also note there are external battery options available for USB, you could take mobile power without having to carry it in your pocket full time.
Nice thing about the S4 is that it has a factory-authorized wireless charging kit. Just plop the phone down on the charging pad instead of working the USB port to death.
Abacus Model 2.5 | Quad-Row FX with 256 Cherry Red Slider Beads | Applewood Frame | Water Cooling by Brita Filtration