Welcome to the HP Forums. I would be happy to help you with your HP Envy 7640 Printer. I understand that you are getting C4EB827F error on HP Envy 7640 Printer and you have already done some preliminary troubleshooting.
As you have already tried these steps and still having the issue, I would personally suggest you contact our HP phone support for further assistance. If your product is Out of warranty then remember to choose the paid options to get the tech support number.
They might have multiple options to help you on this.
Please use the following link to create yourself a case number, then call and it may help speed up the call process:
Step 1. Open link: www.hp.com/contacthp/
Step 2. Enter Product number or select to auto detect
Step 3. Scroll down to "Still need help? Complete the form to select your contact options"
Step 4. Scroll down and click: HP contact options - click on Get phone number and phone number appear.
They will be happy to assist you immediately.
Keep me posted for any other assistance.
Cheers