3G and 4G use different technologies, frequencies and standards
The ITU relaxed 4G specs to allow faster service, even those still using 3G technologies as long as they move towards the 100Mbps / 1GPS standards.
(Sorry as a newb, I can't post links until 100 posts)
Google 4G wiki
In March 2008, the International Telecommunications Union-Radio communications sector (ITU-R) specified a set of requirements for 4G standards, named the IMT-Advanced (International Mobile Telecommunications Advanced) specification, setting peak speed requirements for 4G service at 100 megabits per second (Mbit/s) for high mobility communication (such as from trains and cars) and 1 gigabit per second (Gbit/s) for low mobility communication (such as pedestrians and stationary users).
Since the above mentioned first-release versions of Mobile WiMAX and LTE support much less than 1 Gbit/s peak bit rate, they are not fully IMT-Advanced compliant, but are often branded 4G by service providers. On December 6, 2010, ITU-R recognized that these two technologies, as well as other beyond-3G technologies that do not fulfill the IMT-Advanced requirements, could nevertheless be considered "4G", provided they represent forerunners to IMT-Advanced compliant versions and "a substantial level of improvement in performance and capabilities with respect to the initial third generation systems now deployed".
HSPA+, or Evolved High-Speed Packet Access, is a technical standard for wireless, broadband telecommunication. HSPA+ enhances the widely used WCDMA based 3G networks with higher speeds for the end user that are comparable to the newer LTE networks. HSPA+ was first defined in the technical standard 3GPP release 7 and expanded further in later releases.
HSPA+ provides an evolution of High Speed Packet Access and provides data rates up to 168 Megabits per second (Mbit/s) to the mobile device and 22 Mbit/s from the mobile device. Technically these are achieved through the use of a multiple-antenna technique known as MIMO (for “multiple-input and multiple-output”) and higher order modulation (64QAM) or combining multiple cells into one with a technique known as Dual-Cell_HSDPA.
The 168 Mbit/s and 22 Mbit/s represent theoretical peak speeds. The actual speed for a user will be lower. In general, HSPA+ offer higher bitrates only in very good radio conditions (very close to cell tower) or if the terminal and network both support either MIMO or Dual-Cell_HSDPA, which effectively use two parallel transmit channels with different technical implementations.
The higher 168Mbps speeds are achieved by using multiple carriers with Dual-Cell_HSDPA and MIMO together simultaneously. 
The technology also delivers significant battery life improvements and dramatically quicker wake-from-idle time - delivering a true always-on connection. HSPA+ should not be confused with LTE, which uses a new air interface based on OFDMA technology. HSPA+ is an evolution of HSPA that upgrades the existing 3G network and provides a cheap way for telecom operators to migrate towards 4G speeds without deploying a new radio interface.