Update 10/28/13 10:30pm PT: The update is rolling out now (thanks, bdh and ohbuckeyeman!):

update 4.3update

Samsung has added details about the upcoming I545VRUEMJ7 software update that will, among other things, make the device compatible with the Galaxy Gear. That means anyone who ran out and purchased the watch before discovering that it was only compatible with the Note 3 can now breathe a bit easier (I'm sure at least one person did that, right?). Oh, and they're bundling this with a nice little thing called Android 4.3.

SamsungVerizon

The OTA is set to launch today, which matches the timeframe we saw in Samsung's American Android 4.3/Galaxy Gear update schedule leaked last week. The update includes Android 4.3, adds support for Galaxy Gear, updates Group Play to version 2.5, adds support for Samsung KNOX, and enables Band 4. This last part is a doozy, as it means the S4 will now be able to connect to Verizon's recently deployed AWS LTE spectrum capable of producing significantly faster download speeds in areas with heavy usage.

What's new:

SUPPORT HAS BEEN ADDED TO:
• Blocking Mode feature in Settings Menu
• Enabling NFC feature before using Group Play
• Samsung Knox, Security Enhanced for Android

CHANGES HAVE BEEN MADE TO:
• OS has been upgraded to Android Jellybean 4.3 OS
• Starter Mode renamed to Easy Mode
• Viewdini stub application has been removed (note: customers
who already downloaded the full application will not lose it)

IMPROVEMENTS HAVE BEEN MADE TO:
• Weatherbug application “auto-refresh”
• Mobile Hotspot now works with the Data Usage Meter
• Video app crashing has been resolved
• Call audio issue while on a call and taking a picture have been
addressed

Non-S4 owning Samsung fans, hold tight. Updates for other Galaxy devices, including the Note II and the S III, should arrive within the next month.

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Source: S4 Software Update Page, Press Release, PDF