There are a couple ways you can do this. You could use the Simulation.CurrentPositionAge function as part of your sell signal (this function only works in the System Tester), such as:

Simulation.CurrentPositionAge >= 4 AND {Sell Signal}

You could also use an Inactivity Minimum Change Stop in the Stops tab (also only in the System Tester). Set a value that will likely never be met, i.e. 100%, and set the periods to 4. This would mean that unless the security price doubled in those 4 bars, it would exit the trade.

You could possibly do something like this:

{Buy Order}
BC:={Buy Condition};
SC:={Sell Condition};
TRADE:=If(PREV<=0,If(BC,1,0),If(SC AND BarsSince(PREV<=0)>=3,-1,PREV))
Cross(TRADE,1)

{Sell Order}
BC:={Buy Condition};
SC:={Sell Condition};
TRADE:=If(PREV<=0,If(BC,1,0),If(SC AND BarsSince(PREV<=0)>=3,-1,PREV))
TRADE=-1

Edit: Sorry there were some initial accuracy issues so make sure to use the latest revision of these.