4W per gallon is definately more than enough. I've tested my riccia, and they pearl even at 2W/gallon.

There must be a balance between CO2 and light. Too muc light and NO CO2, there'll be slow growth. Too much CO2 and too little light, slow growth too.

You need to find a balance point By the way, my riccia is growing under 1.5W/Gallon w DIY CO2. Horribly slow though. After 2 weeks, it grew only like 0.7cm haha. But I'm a patient guy

Whereas, in my other tank (2W/Gal and Cylinder CO2) after 2 weeks, it grew about an inch (4 times growth rate).