Newly sold Samsung Galaxy S4 models are Spark capable. Make sure you are getting model: SPH-L720T
If you buy your Sprint GS4 second hand or a refurbished model, you will most likely receive a single band (B25) model.
This is a great question and sorry for any confusion. The information GRAPKOKI1 provided is accurate. The Samsung Galaxy S4 recently received a redesign to have Tri-Band capabilities but older versions of the S4 will not have this feature available. To determine which model you have sign into your Sprint online account and scroll down to your phone line which the model will be displayed.
Sprint Social Care Team
The only way to get the new S4 triband would be to order it from sprints website. But also note that the new triband phones do not do SVLTE or SVDO. the SPH-L720 frequency bands are 800-adv bc10/1900 MHz (cdma), 1900MHz (LTE b25), 850/900/1800/1900 MHz (GSM) and 850/900/1900/2100 MHz (UMTS) it will do SVLTE but no SVDO. hope this is what you were looking for.