Shall I add a RAM or change the PC?

do a fresh windows install if you are running windows. Make sure you are letting it breate. The vents are typically on the bottom and if they are blocked aznd it can get fresh air it will get hot.

Is the fan working? The blow out the hot air from the cpu?

4gb of ram is plenty for web browsing and typically some work depending on what work is for you and what programs etc