they say 5-6 watts is excellent.
multiply the gallons u have by 5 and u should get what u need.
u have say 30, so u would need 150watts. 24x4 ur only working with 96watts. my biocube was 30 and all i had was 2 36watt lights total of 72watts so i then upgraded to 144watts and the coral LOVE LOVE IT. the 96watt lights your looking at would work but u wont support coral that need strong lighting i rec to get strong lighting so that way coral at the bottom of your tank would feed nicely too juss keep looking, u dont want to later change ur mind about ur lights and upgrade just save money now and buy the proper lighting instead of skimping u know??. hope this helps im not a pro but i try an help. cheers!