Can I implement 8 DDR2 memory controller in a S4?

I have not done it. May I ask which part you'll be using? Basic things to remember:

1 - I/O count

2 - DLLs. You'll have to share the DLLs to get up to that number

3 - PLLs. Unless you get creative, you're going to consume 8 PLLs. If you implement the regular DDR2 controller rather than the High Performance controller, you might be able save on this.

Do you really need 8 separate controllers or could you have 4 controllers with double the width? SoPC builder and the Avalon fabric makes it very easy to connect multiple components to a single DDR2 controller.

Jake