Software development requires continuous learning it may be new technology/language methodology or a complete new platform so Why only 4 Years , In my opinion as software developer you are a student always, you need to continuously increase your knowledge and its practical implementation; technology comes and goes so you need to be adaptive.Yes it always takes some practice experience to build something valuable but you can always start with something very little and working and then build as you improve your knowledge and skills. So be open minded learn something new every day and build valuable things.