If you want to use all four you could use two on the sides of the tank and build to into the rockwork. It depends if you would like to set them up for a visual aspect or not as well.
You could have the two 6055's at either end of the tank and have the two 600's built into the rockwork using the stream rocks (6200.25). They can be used for the 6000's but there isnt currently a nanostream rock available, hint, hint. This will still allow you to control your flow rates and still get enough circulation within the tank.
The 6055's arent as distracting so they wont stand out on the sides and your 6000's wond stand out when they are built into the rockwork and coraline algae covers the stream rocks.
Cheers
Ryan