I think it's the engine... or I need to get her tuned again.
I'm running EJ20G (closed deck) with bigger injectors and turbo (TD05H-20G). I don't hit full boost until 4-4.5k. When I had the stock turbo (TD05H-16G), it spooled pretty late too around 4k.
A totally different animal compared to the 06 STi I had back in the day, when I could get full boost (21psi) by 3000 rpm haha. Then again, less displacement and a bigger turbo so I'm not surprised.