I would sugest you try it in three different periods of time. For instante, for EURUSD from April 94 till October 2000 when it was a downward period, from October 2000 till june 2008 when it was an upward period and from Auguts 2008 till jan 2014 when it has had more or less a lateral movement period.

For what JCL is telling in similar question, make sure you get 50-100 trades at least for each degree of freedom when you do the WFO. In your case you have 4 degrees of freedom to optimize, so after WFO you should have 400 trades for the system to be reliable. May be you can try to increase the number of cycles to increase the number of trades.

Also, closer to the present date, better. That is, he says that a system has 5 years lifetime. Though, I am not giving up the search for the holy grail...a system that works similarly in all periods of time, either up, down or lateral movement.