The G4 stands for a two pin plug with a slight difference. The two pins are not the usual kind, but are just the end terminals for the positive and negative of the bulb. These bulbs cannot be plugged into just any socket and need some work before they can be used.
Generally the G4 bulbs are used when you want a small light that you want to solder into a board for whatever reason. Now with the small sized LEDs that are available in the market the G4 LED has actually become very common, starting with just connecting them to two wires and attaching them to a small 1.5 Volt battery to give you light for some toy to making larger torches and such.
The advantage of these lights is their small size. A single bulb can be just half the radius of an average pencil and creating something with them is no big deal. In fact if you take a small battery and holding one of the terminals to one end, if you connect the other terminal to the other side of the battery with a wire, you have yourself a small flashlight.
There are however larger bulbs too with a higher discharge, and you can have many more of these small bulbs incorporated in them. These bulbs are usually used in decorative light fittings where you don’t want the kind of light given off by a normal bulb. These bulbs are so tiny that they really do not have much of a light output, and are very good if you want an indicator bulb that will let you know if something is happening or not.
There are no holders for this kind of bulbs which means that you should be handy with electrical tools. You however do not need much and if you take apart some of the cheaper toys you will see how simple it is to make use of one of these lights. The G4 stands for a size and not for the output, and you even get miniature halogen lamps in this size. These lights are used for those places where you want a bright light, but do not have space for a normal sized bulb. A simple example would be in Christmas decorations where you need small yet bright lights. With the advent of the G4 LEDs this has not only become simpler, but has also become cheaper because these bulbs cost less than a dollar each, and many times cost only 1/5th of one dollar.
Some of the cool things that you can do with the G4 LEDs are to install them under the hood of your car so that you get a bluish light coming from under the hood. If you know your way around a computer, you could solder them to your comp so that you get whatever color light you want to. Some places even give you a slot where you can just push these bulbs in and that’s it. They are held in place and are fairly secure, and since they are very robustly made, there is no problem with them breaking.
There are many electrical appliances that give you the option of using large G4 bulbs instead of the normal incandescent ones, and newer lights are coming out that use this system more and more. They are just as effective as a normal bulb and holder, and because LEDs are so small, you can pretty much make them as small as you want to. The smaller LEDs can actually go hours on just one 1.5 volt battery while the earlier bulbs would pretty much not last beyond 15 to 30 min.