Why is it that I am getting 404s when the profile is valid?

Proxycurl only scrapes public Linkedin profiles. Some Linkedin profiles do not have public profiles (due to privacy settings). And if the public profile does not exist, then we are not able to scrape it. This is when we return 404.

You can verify that a profile is missing a public profile by Googling that profile. You will see that it is not indexed by Google and not present on the search result page.