A 4 cylinder with 250 horsepower doesn't make any sense for a Supra. First of all, the GT86 is already a 4 cylinder, and with any luck, the second generation should be outputting at least 250 hp (and it probably needs to with all of the upcoming competition targeting it).
What makes more sense is an N/A 6 cylinder (whether V6 or I6) with similar power to the 370Z (330+ bhp), and a single or twin turbo 6 cylinder with similar power to the Corvette and GTR (450+ bhp).
I could also see them releasing a third version after a couple of years into development, much like the way Chevy does with the Corvette and ZR1, with either a bigger turbo(s) or an additional hybrid motor(s), outputting over 500 bhp.
It really all depends on what their competitive targets are for this car, and what they need to do in order to compete with those cars. In that sense, a 4 cylinder with 250 hp just doesn't make any sense. I realistically cannot see the Supra coming with a powertrain that delivers under 300 bhp in this day and age. This isn't 1993; even the V6 Mustang is making over 300 bhp these days.
But honestly... I'd like to see the FT-1/Supra come with a n/a V6 base model, n/a V8 mid-range model, and twin turbo V8 top-end model.
EDIT: Small V8's btw, like 4.2L, but with all of the best engine technology available; DOHC, Hybrid Injection (PI/DI), VVTL-i, the works.