Here is what one speaker designer has to say about designing 4 ohm speakers
"Thiel speakers have long suffered a reputation for being power hungry. Jim Thiel admitted that most of his speakers sport 86-87dB sensitivities and are 4-ohm loads. He could easily transform them into 8-ohm loads, but he prefers to avoid the concomitant loss of sensitivity by 3dB. He also uses 4-ohm loading since most well-designed solid-state amplifiers will double their output power into such loads. Why purchase this amp capability only to leave it unused?"
taken from http://www.soundstage.com/yfiles/yfiles200107.htm
"Thiel speakers have long suffered a reputation for being power hungry. Jim Thiel admitted that most of his speakers sport 86-87dB sensitivities and are 4-ohm loads. He could easily transform them into 8-ohm loads, but he prefers to avoid the concomitant loss of sensitivity by 3dB. He also uses 4-ohm loading since most well-designed solid-state amplifiers will double their output power into such loads. Why purchase this amp capability only to leave it unused?"
taken from http://www.soundstage.com/yfiles/yfiles200107.htm