Inboudn/outbound means that on 2 of your travel days, you can also travel in your country of residence. Also see this page.

Your plan looks like you'll travel in Germany on 3 days. So check on which of those 3 days a normal ticket is cheapest and use Interrail (4 days/1 month) for the other days. 

I think you should also check if buying normal tickets for all days would be cheaper than Interrail plus reservations (you'll need them to and from Paris, unless you have a lot of time).

Thank you that helps a lot.

So if we decide instead of travelling to Berlin we will go from Brussel to Rotterdam and then from Rotterdam to Amsterdam it will work out, am i right? 

And would it be possible just to buy a normal ticket between Paris and Brussel and use to interrail pass for the other four days?

Sure.

Sure that's possible. The journeys you do with Interrail do not need to connect with each other.

A cheaper way to avoid Thalys between Paris and Brussels (reservation 25€) is to take a TGV to Lille (10€) and then a normal intercity (reservation-free) to Brussels. Or travel via Aulnoye (reservation-free). There are some options, but they also take a bit more time.