My legs stopped growing. I am doing 4 exercises.
Legs Extensions
Leg Presses
Leg Curls
Barbell Step-up
4 sets in each with 12 reps.

Please give me an advice.

Thank you.

no squats dude? do you deadlift?

too much isolation and no compount movements in there IMO..

My leg workout is simple, straight forward and i have had to stop training legs to let everything else catch up!

squats
stiff leg deadlifts
calves

and thats it... drop the machines and pick up a bar dude 8)

try simple single factor training bud.
using deads and squats as andy says..
adding weight on a consistent basis will be the shock youre muscles deserve.
as you approach your PB drop the increments to as low as 1kg.
if youre diet is good and youre getting enuff REST youre body will adapt to the small weight increments for far longer than trying to add what most gyms have as a minimum weight increment ie 2x1.25kg plates.
this has been one of the biggest facotrs in my progress.

natural or enhanced the principles are the same.

get big first then fcuk about with isolations when you need to shape the muscle.

Go basic. Less exercises. Heavy weight. Train till you puke.