My period last for 4 days is that normal? Some people period are for 7 days?

Yes. It varies but if yours is 4 days, then that's you and it's normal.
Variation. Each person differs because their body is an elaborate system of tissues and hormonal stimulation. The often cited normal range for menses is 3-8 days, but what your system does for you is your normal pattern. When you vary significantly from your normal is when you should pay more attention to want may be going on.