The short answer is no, the iPhone 4 and iPhone 4S are not 4G phones.
At least they're not when by saying "4G" you mean the 4G or 4G LTE cellular network standard (the successor to the 3G standard used by the iPhone 4 & 4S) that most phone companies are referring when they say 4G. To understand the confusion, though, requires understanding what people mean when they say something is 4G. The confusion happens because there are two different meanings for the term 4G.
4G = 4th Generation Cellular Network
When most companies, and some people, talk about 4G, what they mean is a phone that is compatible with a 4th generation (i.e. 4G) cellular phone network.
4G networks, also called LTE Advanced or Mobile WiMAX networks (among other names), are next-generation wireless networks used by mobile phone companies to transmit calls and data to mobile phones. Just like the current 3G name means that something is a third-generation network, 4G networks are newer, more advanced, 4th generation networks that will replace 3G networks.
4G networks are faster than 3G networks and can carry more data:
- 4G network speeds: up to 1 GBit/second download; 500 Mbits/second upload
- 3G network speeds: up to 14.4 Mbits/second download; 5.8 Mbit/second upload.
While some 4G networks exist in limited implementations or implementations limited to some areas, most large-scale 4G network deployments will take place in 2012.
4G = 4th Generation Phone
The confusion about this question comes as a result of people sometimes using the term 4G to mean all fourth generation products generally, not 4G networks specifically. The iPhone 4 is, as the name would suggest, the 4th iPhone model, making it the 4th generation iPhone. But being a 4th generation phone is not the same thing as being a 4G phone.
The iPhone 4 Is Not a 4G Phone
4G phones are phones that work on 4G networks. The iPhone 4 is not compatible with 4G networks (the same is true for previous models). Because the iPhone 4 works on 3G and EDGE networks only, the iPhone 4 is not a 4G phone.
Neither Is the iPhone 4S
The iPhone 4S can download data at a faster speed than the iPhone 4 (up to 14.4 Mbps). While this isn't 4G speed, some cell phone companies may promote the iPhone 4S as being a 4G phone or close to a 4G phone. Technically, this isn't true. As noted above, 4G requires a particular type of cell phone network and chips in the phone. The iPhone 4S doesn't have this chip. Some of the phone companies that offer it in the U.S., most notably Verizon, have extensive 4G networks, but the iPhone doesn't take advantage of them.
How About the iPhone 5?
4G phones are notorious battery hogs and don't deliver much use before needing to be recharged again. Giving the iPhone a long battery life is very important to Apple; battery-life concerns are probably what kept 4G LTE support out of the iPhone 4S.
Thanks to the iPhone 5's larger dimensions, and engineering changes by Apple, the iPhone 5 both offers 4G LTE networking and solid battery life (Apple claims that the battery life is as good as the 4S, or better in some cases). So, if you want to get 4G LTE for the fastest cellular data experience, pick up an iPhone 5.