I was going to suggest 4 (if by Sea of Green) but I see this has been addressed by others. Your decision should be about best results rather than how many plants.

I personally would use the Screen of Green in this small a space and grow only 1 plant (I have had great success doing this exact thing). This is primarily due to the limited hieght involved. However scrog can be labor intensive and does have a bit of a learning curve so if this is your first grow you may want to try Sea of Green on this one.

Either way the use of haircuts will be necessary for best results in a small space (light penetration issues). Good luck and welcome to the addiction of grow your own.

BTW 600w is huge for this 4sqft area so watch out for heat buildup issues. Ventilation can be critical with regard to controlling heat. I have used 250w HPS with good results in this small an area so maybe 400w would be a reasonable comprimise if you used a cool tube. 50 watts per sqft (HID) is a primitive but widely used measure of how much light to use.