Did your doctor advise fasting? Are you a diabetic (wondering since you’re referencing blood sugar numbers)?
Fasting has a lot of benefits but 4 days is a long time and could be harmful if you have other medical conditions. You may not be able to go about your normal activities (driving, physical work, etc.), and if you have diabetes it can get extra dangerous. I’d recommend speaking to your doctor about this.
People typically work up to a 4 day fast, don’t just dive into it.
There’s also a lot of pseudoscience around fasting. A lot of good science too but I’d make sure you have a good basis before you commit to 4 days as a treatment for something