The FX5 does create a lot of current.
How long is your 400L tank?
What firenzenz says is right, the FX5 does create a lot of water current, but what sort of plants are you going to have in there?
I have set up a 5ft tank with discus and plants for a customer who had an fx5 on the tank, and the water flow was negligible because of the way the outlets and plants were positioned.
Use plants like sag. subulata that get tall and bushy, and tiger lotus. Driftwood strategically placed also will act as a barrier to water flow, so it can be done- just depends on how you want to do it.
Do not discredit having 2 filters on the tank either, although 2 405's brand new will be much more expensive than an FX5.