I often hear confusion from people who are looking for a new phone surrounding the differences between 4G and 3G, and between the various versions of 4G. I also hear people who are sure that the iPhone 4 is a 4G phone. I am going to attempt to shed some light on this.
This is my opinion:
To start with 4G is just a marketing term, it really doesn't mean anything because there is no true standard for what 4G means. If you are choosing a new phone it may be important to have fast data and if you are planning on staying with your current carrier and signing a contract - go with the 4G phones as they will be the fastest phones on the carrier (perhaps not immediately, but it is very likely that in your 2 year contract that they will be). If you are changing carriers or thinking about starting service with a carrier the differences in 4G technology can be very important.
So what does the G mean anyway?
G in this case means Generation. The first generation of wireless technology can be called 1G. These were the phones of the 80s and were analog. The reign of 2G phones begin in the 90s and offered improvements like data (text messages) and digital communication (instead of analog). The 2G network is still around and if you have a GSM phone (T-Mobile/AT&T) then you may see an E instead of a 3G from time to time depending on your coverage this is the EDGE network (a 2G network). On a CDMA carrier (Verizon/Sprint) you may notice a 1x on the top in low coverage areas. The EDGE and 1x networks are actually improvements beyond the original 2G network, sometimes being called 2.5G or 2.75G.
3G phones became available in 2001 with greatly improved data rates, but unlike previous generations of the technology there was not truly a standard or bench mark for what speeds a provider had to operate at to call their service 3G. Like the 2G standard before it, there are half-steps before the next generation. One of these half steps is HSPA+, which is a 3G technology that augments the speeds of 3g offering 2-4 times the 3G connection speed currently (with a max connection somewhere around 56Mbps).
So to be clear the "G" really just means generation, and depending on the technology in the phone this is how fast your mobile data will be (how fast webpages load and files download). This has little effect on your voice quality (the 1G phones actually have the best potential for voice quality due to the way analog signals can travel through the air). If you are not looking for data on your phone, this may not matter. Websites are typically able to be loaded quickly regardless of total Mbps down (the speed of your connection) and may be more effected by latency (the time it takes to connect to the server).
4G vs 4G
Okay, so this is where it gets a bit tricky. Sprint has been advertising their 4G network for a long time. Nearly a year ago (the EVO was released in June of 2010). Sprint launched their WIMAX 4G service, which is capable of 128 Mbps (no phone is really achieving this speed currently, and it would be hard press to find one that is averaging more than 15Mbps - which is still faster than most DSL customers). WIMAX is technically an IEEE network standard like the G or N network that is likely in your house. The downside to this is that it makes it difficult to have long range from the towers, and is likely to only have city wide coverage for select cities (even into the future). Sprint heavily marketed their network as "FIRST". As they were the first to 4G coverage.
The reaction from T-Mobile, who had launched their HSPA+ network around the same time as Sprint, was to switch the branding from HSPA+ to 4G. This allowed them to work off of the marketing groundwork that Sprint had already laid. HSPA+ is currently similar in speeds to the Sprint WIMAX, but the upward maximum for the technology is only at about half.
AT&T recently launched their 4G network, which is also HSPA+, and plans to do an LTE network in the future. LTE is the type of 4G network that Verizon has launched. The benefit to this is that it has an upward maximum of 100 Mbps (and with LTE advanced it could be as much as 300Mbps). Verizon's LTE is running on the 700MHz band that analog TV ran on (before the digital conversion). This means that they will be able to put towers across the entire country an blanket us with these amazing speeds (if they follow their past 10 years of coverage expansion). LTE is also an improvement that allows for voice and data at the same time (old iPhone commercials call out AT&T as a better network because of this feature that Verizon lacked).
What do I recommend?
In the end I currently recommend Verizon or Sprint's version of 4G if you are in the market for a phone that will be the fastest in the future. Also, they both currently offer unlimited data which is something that will likely become a must with unlimited terrestrial broadband going to the wayside. With that said, I think that AT&T is a great company and if you live in the right area it can be the perfect provider. Also, with the looming purchase of T-Mobile, AT&T will grow into an even bigger provider and who knows what they will have the potential to offer!