Why my memory usage is so high? (2.8GB out of 4GB immediately after wiping clean)
It looks pretty normal to me. 

Web pages each use memory &amp; it soon adds up. Right now your Activity Monitor is showing some compressed, but no swap used; which means everything is still happy.
Having only 4GB of RAM these days means you've just always got to keep an eye on it, but so long as memory pressure stays out of the red, everything is nominally OK.

The modern paradigm is that empty memory is wasted memory, so the machine will fill the memory as it needs, to enable a faster response if any of that used memory is needed again. It is far faster to re-use memory than it is to empty it &amp; refill it with the same data over &amp; over, so it hangs onto used memory it thinks it is likely to need again.