HSPA+: This is one of the earliest "4G" technology and it's adopted by US carriers such as, T-mobile and AT&T. Although it was marketed as "4G" by these carriers, it's really an improved version of their existing HSPA 3G network. At 84Mbit/s download and 22Mbits/s upload, this technology is theoretically slowest amongst the "4G" networks. Strictly speaking this shouldn't even be called 4G by many recognized standards. But for those who are eyeing the recently announced iPhone 4S, this is the fastest standards it will support. Since AT&T is the only carrier to offer the standard, theoretically the AT&T iPhone 4S will be the fastest amongst the three carriers.
WiMax: A standard offered by Sprint in partnership with Clearwire here in the US, this is really the first 4G network deployed. Sprint's HTC Evo 4G is the first mobile phone to adopt this standard, making it the first 4G phone in the US. At the top theoretical speed of 128 Mbit/s download and 56 Mbit/s upload, making it faster than HSPA+. However, Sprint has recently announced the transition over to the LTE standard. From what we see, it seems that WiMax might be a dying standard here in the US.
LTE: This is the big three letter words in mobile technology. Verizon recently started their test run for this standard with AT&T following suit, even Sprint recently announced its transition to LTE. This as it appears would be the next standard of mobile network. The theoretical speed with the anticipated update, clocks in at 1 Gbits/s download and 100 Mbits/s upload, this is the fastest standard available stateside. However, since it is a relatively new standard, it still hasn't been widely deployed yet, so your experience may be more unstable.
Now that you know all 4G are not created equal, you'll be able to make a more informed decision when choosing your wireless carrier. Let us know what you think in the comments below! Check out our detailed post on Clearwire's WiMax 4G.