As a former WERA roadracer with 40 years experience on bikes, I'll suggest the Suzuki SV650 as your starter bike.
The guys above do offer good advice. The way I've always said it is: "you don't learn to fly in an F-16; you don't learn to drive in an F1 car; don't try to learn to ride on a GP bike--aka R1, GSXR, CBR, etc.
The SV has a much broader spread of power than any IL-4 engine, making it easier to ride; and, IF RIDDEN WELL, can simply SPANK squids on IL4's in the twisties! The fours are all high RPM engines, and require much more skill and work to keep em on the boil.
The other advantage of the SV is that you won't outgrow it nearly as fast as you will the Ninja 250 or 500.
Whatever you decide, DO NOT LET TESTOSTERONE DICTATE YOUR CHOICE!!!!! Use your head, think smart...you can get killed on a bike without doing anything wrong at all! Come to think of it, 'use your head, think smart' is the best advice I can offer a new rider!