The Clark board uses a 3 watt amp on board, due to power draw 8 ohm would be best BUT 4 ohm will not have any kind of discernable difference to your naked ear.
If you want any advice on your speaker here is some in a nutshell
Get a speaker with as high a watt rating as you can fit in your tank I suggest 8 watt being your minimum - this is so it will handle the power ( goes double if you use Clark's new 6 watt amplifier http://shop.immortalhobbies.com/Clar...9962310135.htm )
Get a speaker that carries from 20 hz to 20,000hz the higher and lower the hz the cleaner and crisper the sound. No use having 6 watts of power if everything sounds the same due to distortion

For an improvement in sound the best thing you can do is drop the potentiometer that HL uses ( volume control ) and improve on that, 10K pots are the best.