They will be okay as long as they aren't too close. Seeds/clones are supposed to be about 3-4' away, and then over time slowly close it in on about 2'. When veg growth takes off, due to the fact that you have a cooled hood, you can get that light as close as 12" while still maintaining a safe distance for heat/burn caution. When you get it to the point where they are loving 12", you will notice very close internodal growth, but be sure to raise it an inch every couple days of veg, as they will sprout lots of new growth in that span.
3'x3'x4' sounds WAYYYYY too small to have 16 plants going in. The easy ryders aren't autos, are they? If they are, then you could possibly fit that as long as they don't branch out much, and some netting when they start to flower would help.
As for the 600watt, that is a great quantity of lumens to disperse over that space, and you should see some really solid results if you take good care of them. You will have the cooling taken care of by using an inline fan on your hood. Making sure that you do a controlled feeding schedule so that you do not burn your girls, along with oscillating fans for good airflow. Proper nutes & airflow will make a big difference in yield, potency, & overall quality of bud. If you really want to try and get as much yield as you can out of it, you should look into a CO2 tank and a regulator unit. All that should cost you around $250 if you want to spend the money. Lots of growers report a minimum 20% increase in yield that should more than pay for itself, and then there are some that claim to CO2 doubling their yields. I saw a big difference when I started using it, but didn't do a good job logging what my first few grows without produced, so I was left guessing and really don't know how much of an increase I saw.
Hope some of these tips help! =)
+rep? what? haha