Yes you can run a 400 watt bulb.
Yes you can dim your 600 watt bulb to 400.
It's up to you,either choice works.
Changing the bulb or changing the setting on your ballast is not gonna make or break your grow.
A 4x4 tent would be a good option for your light.To me a 3x3 space is a waste of 600 watts.
By the pics your issue isn't lighting.
You haven't mentioned your feeds and feed schedule.And what about your humidity?
Yes you can dim your 600 watt bulb to 400.
It's up to you,either choice works.
Changing the bulb or changing the setting on your ballast is not gonna make or break your grow.
A 4x4 tent would be a good option for your light.To me a 3x3 space is a waste of 600 watts.
By the pics your issue isn't lighting.
You haven't mentioned your feeds and feed schedule.And what about your humidity?