Yep, flowering for sure. ITS A GIRL ! Congratulations !

However, I'm very confused to why its just 5cm in diameter after 4 weeks of veg ! In my grow my plants were monsters after 4 weeks of veg, I even supercropped them and they are huge now after 6 weeks of bloom.

Something isn't right there in your grow room, I'm guessing you are growing under a very small light.

After 4 weeks some autos will bloom. It's a normal time to bloom for autos. you need to up the light source to get the plants ready (big) for their time clocks to kick in.

Have a look at my set up here under "indoor growing" - "testing the pro-grow 260 LED"

Or just follow this link LED Logger

Every stage of my grow is documented and has pictures for every update, hope it helps in some way.

Take care