When a petrol engine idles the throttle is closed. That creates a vacuum (under pressure).
Whether 0,3 bar (4.5 psi) is exactly correct for your engine on idle I don't know. But it's at least close to what it should be. In general: the more you press the throttle pedal, the higher this reading will be.

Given the fact that your knowledge is limited, isn't it time to contact a professional?
Before you start replacing perfectly good parts that cost quite a bit of money.
Let's face it, for instance when I ask you to measure the fuel pressure you don't have the equipment to do that.