So if I wanted to trade two Xbox one games for the same ps4 games is that possible or do I have to pay some charge ?

It depends if the games you're trying to trade are equal or more then the price for the Ps4 game if total is under the price of the game or games you are trading then your going to have to pay the remaining amount to get it

They more than likely cost more for the newer system games so assume you will have to pay something for the exchange.