Why stuff around with a 5th injector when you can run evo 1-3, 510's (or evo 4-8 560's) and a chip to compentate (if you have an eprom (chippable) ecu). Even if you have this to buy an ecu, chip and injectors, it would probably not cost more then tring to tune the extra injector and be safer and more reliable.
I personally wouldnt bother with it.
I think a chip and injectors would be a good start if you want to run more boost or change turbo's. And if you want to get more serious later you can use the eprom ecu and injectors and buy ecmlink, and just progress the system. Or go a piggy back ecu/ full ecu ect. But this would be a much better way to start playing with the car and you can progress form there. Rather than put time and money in a limited system. And there is less chance of double buying parts, and also the parts your using are able to be easily sold when you outgrow them.