G4 Bulbs are commonly referred to as Halogen or Xenon Bulbs. They have 2 pins on the bottom approximately 1/2" - 1" long. They are glass and generally get hot within 1 minute of being plugged in. They commonly come in 5 watt, 10 watt, and 20 watt variations.
LED G4 bulbs come in a variety of models depending on the application. They still have the 2 pins, but can shine either in 1 direction, or in all directions. There are also variations to match the various light output of the 5 watt, 10 watt, and 20 watt variations.
For halogen bulbs, use Warm White to replace and keep the color to a similar yellow tone.
For Xenon bulbs, use Cool White to keep the white with blue tone light.