Sounds like you might have 2 strains there. I'd move the 400 to center over the taller ones. Being moved directly over them will give them a little more light, & being taller, they'll be closer. Then I'd put the 600 over the shorties, which of course will give them more light, since you can't lower the light to them without burning the taller ones. ~~~ PEACE! ~~~

Your area is small enough that bringing in the extra light will boost lumens to all 6 plants.