Actually, I sent Anzo an email this morning to ask them and they just replied this:
"the G4 comes with SMD and the G3 comes with LED"
Not sure exactly what they mean as SMD is a type of LED, but I guess it means that the G4 has improved/updated leds mounting/hardware.
http://www.jledlight.com/What-is-SMD-LED.html
What Is SMD LED?
SMD LED stands for surface mount LED. Specifically, it's a light-emitting diode that is mounted onto and soldered onto a circuit board. An SMD LED is quite small since it has no leads or surrounding packaging that comes with a standard LED. This means it's best handled, not by a human, but by automated assembly equipment. An SMD LED also has a wide viewing angle, thanks to the fact that it does not have the standard LED's epoxy enclosure that focuses the beam.
Benefits of SMD LED
The SMD LED gives off very little heat. It also has a low voltage and current requirements. Like a standard light emitting diode, a surface mount LED gives off almost no heat. It also typically has similar low voltage and low current requirements. They are commonly used to indicate device status on computer motherboards, routers, hard drives, USB flash drives and any other application where physical space is at a premium. Computer motherboards, hard drives, routers, flash drives and other applications where there's no room for larger technology are also common uses.
Others include LCD display backlighting, keyboard lights, and pushbuttons. They've also been used for instrument panels in aircrafts. A few manufacturers have created an SMD LED lighting solution by combining it with a resistor, small circuit board, lens and cable to provide and bright, cool LED light that runs on just 12 volts, and which is tiny enough to be mounted almost anywhere, (for instance, to light a cup holder).
There are other, large-scale applications, too. For instance, they are used for indoor display screens. SMD LEDs work well because you can arrange a large number of diodes in tight groups of red, blue and green, generating a large variety of colors. This screen technology, which is now found in stores and malls, is now also popping up in large outdoor displays.