4.5 GB of system files? I can guess it's been more than 10 years you have been using your windows phone.
What you can try is, if you can afford it, doing a hard reset, to bring back the space taken by the system files to a normal of 3 GB.
From the phone go to: Settings > System Setting > About > Reset your phone. Keep the phone plugged in till the process is completed.
Worrying about your messages on Whatsapp? There are OneDrive data upload systems both in system settings for a lot of apps. Some might not have, but the messages on Whatsapp can be backed up from a service in 'Whatsapp's settings' itself.
Having trouble in backing stuff up? You can even take offline backups of individual apps. This is a bit difficult to do, but yet if you want to know, comment below.
This answer was related to Windows 10 OS. If your phone is Windows Phone 8, the answer would a bit different. Comment below for me to change it. If this answer helped you, do give me an upvote, and mark this as the answer. Thank you!