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.
I have Acer laptop that run on Windows 10 (i5-7200U, 2.5 GHz, with 4GB DDR4 memory).
Lately, when I open some software (MS words, excel, Zotero, Chrome with many tabs), it run quite slow, sometimes not responding.
I check the memory, it shows 85% (3.3/3,
memory used. Does this means I need to upgrade RAM? Besides that, why slots used showed 1 of 4? Why 1? I did not understand it.

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