you sure you have a b20a? not a b20b/z? the first is a completely different engine and is not compatible with any of the other b series engines.

anyway, whoever's telling you turbo's dont kick in till 4000 rpms is full of shit. a smaller turbo can spool very quickly, but start to max out sooner. at the same time a larger turbo will take a while to spool but makes more power at higher rpms. that's why it's important to pick the right turbo size depending on your ultimate goals for the car.

a stock internal b20b/z should not be revved over 7k rpm, mostly because it doesnt make power any higher. secondly because prolonged use at those rpms will lead to failure.