The fourth beam has been revealed. Spoiler follows:
It's currently unknown if there will be more beams than those 4.
BTW, if you like those sites Mic linked to, you might also find these of interest (You really only need bother checking one... they're all the same as ilovebeams)...
http://www.iloveskis.com/
http://www.ilovekeys.com/
http://www.iloveknees.com/
http://www.ilovesneeze.com/
Nintendo really went site registering crazy.
The fourth beam is known as the Annihilator, and uses both dark and light beam ammo to fire.
It's currently unknown if there will be more beams than those 4.
BTW, if you like those sites Mic linked to, you might also find these of interest (You really only need bother checking one... they're all the same as ilovebeams)...
http://www.iloveskis.com/
http://www.ilovekeys.com/
http://www.iloveknees.com/
http://www.ilovesneeze.com/
Nintendo really went site registering crazy.