Depending on what intake and exhaust system you go with, and how well they match each other, also, you will have to include a re-tune on the ECU if you go with all this, you can get as little as 15 hp, and as much as 30, maybe 35 hp would be doing well. I have seen some get as much as 40, but I think they started a little stronger. ( Remember, not every engine that rolls off the line produces exactly the same amount of hp after break in and wear. You'll probably spend around a grand to two on all that.
And by the way, 4-6k would get you more like 200-250 hp. And you will be well within the limits of the engine. The 2jz I6 is the motor that cranked out 1000 hp in the Supra's.