When I want to buy I get error 004

This error usually occurs because the browser cannot recognize your session cookies, either because they are no longer available, have expired or are blocked. The recommendation is that you restart your browser, delete Cookies and Cache, and additionally make sure that it is not in incognito mode when browsing.

in Chrome
Open Chrome on a computer.
At the top right, click More More.
Click More tools and then Clear browsing data.
At the top, click the dropdown menu next to "Time Range."
Select a period; for example, Last hour or Last 24 hours.
Check the "Cookies and other site data" box and uncheck the other items.
Click Clear data.
If you see this error again, contact support.