4G18 is a SOHC. I think for a safe street tune, can get 180bhp-250bhp range. Torque can get up to 220nm. I think you need a DOHC if you really want to hit 400bhp. My rough estimation :)
Based on a 1600cc NA dohc engine, that is a street going engine (not performance)...a BOT gives the 1.6dohc up to 320bhp (highest that I have seen). But alot things have to be done, not worth the money. Imagine your engine is SOHC.....Swap a GSR better.