A Source Four lighting instrument is an Ellipsoidal Reflector Spotlight, or ERS, made by ETC Lighting.
ERS's are chiefly used for entertainment illumination, e.g. Theatre, Film, and Television. Though the Source Four does, in it's fundamental design, draw similarity to it's leko and Colortran brethren... it supersedes all that have come before.
The Source Four uses a dichroic reflector, which adds to the color temperature (creates a cooler light) and increases lamp longevity by keeping harsher infrared radiation from reflecting back into the lamp's filament.
Of course, being a standard use lighting instrument, it comes with a gobo slot, three-prong gel holders (with clip), and multiple projection angle barrels of 5, 10, 19, 26, 36, and 50 degrees. To clarify: narrower(smaller number) angles would be used with a higher watt lamp to project light from far away, whereas, greater angles could be used to flood an area from very close up.
I just have to say, as a theatre tech, Source Four's are lifesavers.
You don't have to worry about a weak colortran reflector screwing with your gobo projection,
or any other stupid things that pop up with older, low tech lights.