what needs to be done here is to make an EA in ninja trader that will export all trading activity(new orders, close orders, change order, etc) into a mysql database that can be in remote or everything in localhost.
next, an MT4 EA as a receiver that will be reading the database filled by ninja with every tick and find new changes like new trades, trade modification, etc.
It is possible to have the same trades in ninja and in mt4 but by experience copiers are not that easy, to start on ninja and mt4 will have different feeds so a market order in one will not have the exact same price in the other. there is a slippage problem also, when signal goes from one to the other there is a delay. there are also more problems ...
i need to know more specifically how do you want it in order to reduce this problems.
if interested please contact me.