4GB is quite low today. It will run but delays and will pop up not responding messages. NOTHING IS BROKEN HERE. It's just what happens on low memory machines.

Also nothing is broken when you "open some software (MS words, excel, Zotero, Chrome with many tabs), it run quite slow, sometimes not responding."

Just a matter of there is only so much RAM and the OS might work through it very slowly. You can lighten the load with less tabs or add more RAM to help.