AT&T iPhone customers who’ve downloaded the latest update to iOS may have been surprised to see that their phones were displaying the strength of the “4G” network.

Apple has been criticized in the past for not offering any devices that run on the nation’s fastest networks, a criticism that it answered with the 4G LTE-capable iPad it unveiled on Wednesday. But many people may not know that the iPhone 4S is capable of running on AT&T’s HSPA+ network — a technology that AT&T and T-Mobile both use to advertise their “4G” devices. HSPA+ is essentially 3G technology that’s been enhanced to run speeds close to those of 4G networks, but still runs slower than either LTE or WiMax.

The new label likely raised questions for AT&T customers who were affected by the carrier’s latest tweaks to its unlimited plan. The changes stipulate that 4G LTE customers will have their data speeds slowed down after they hit a 5 GB limit for the month; all other customers will see slower speeds after 3GB per month.

AT&T spokesman Mark Siegel confirmed in an e-mail that the iPhone 4S falls into the second category.

There’s actually a lot of debate as to what, exactly constitutes 4G since the International Telecommunications Union relaxed its definition in Dec. 2010. Before then, none of the technologies in the United States — LTE, WiMax or HSPA+ — were considered fourth-generation cell networks. Now, they are.

All this probably doesn’t matter a lot to consumers who just want their phones to load Web pages quickly, but it was still probably surprising for iPhone customers to suddenly see their phones converted to 4G devices overnight.

Related stories:

Apple TV: A simpler interface, easier access to media through iCloud

iPhone 5: The latest from the rumor mill

AT&T increases data plans for smartphones, tablets