Don't need a FMIC at 4psi, and really, to keep it at 4psi, I would say stay with 2.5" cat back - that will keep the internal wastegate working well enough (although not quite good enough for flow). Again, don't need any boost controller if you're keeping the boost down, and of course, depends on the turbo you're running - most likely a T3 class turbo. You need to make sure you can retard timing - at this level, you might be fine with piggybacks like eManage or PerfectPower. Closed loop isn't much of a problem, esp with piggybacks which will kick the stock ECU out of closed loop with the MAP/TPS signal. I guess trading in a WRX would cost more but you would be dead reliable and make the power you want.