Hello
ThÉoPatrizi,
First off, can you please post the error message you get or take a screenshot of it? 4GB shouldn't be a problem for Microsoft Office itself (Only 200-500mb), but that is dangerously low. I recommend you uninstall some unused programs with the Windows Uninstaller.
Go to Control Panel -> Programs, Uninstall a Program.
You should try to keep more than 15gb if possible, this allows for maximum system performance and is considered a "safe" level for your pc to be at.
We will try to help you more when you post the error your getting.
ZeratuelX