There is a dividing line drawn between Jellybean and Kit Kat for apps.
On Jellybean and below, apps used Dalvik for a run environment and at Kit Kat and above, they use ART (Android Run Time).
Your concern would be to make sure the device is upgradable to at least 4.4 Kit Kat to ensure it has the modern ART environment. Other factors like SoC might limit how quickly Apps perform, but I think the S4 is still quite viable.
It might help us check for update levels if we knew which variant you were looking at, such as Carrier branding, 3G vs LTE and any other factors.
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