Me too - and that was the first hit on wikipedia as well:

https://en.wikipedia.org/wiki/Big_Four_%28audit_firms%29

However, if we look at the disambiguation, we find at the bottom of the subsection:

https://en.wikipedia.org/wiki/Big_Four#Companies

"Big Four, the most influential international technology companies Amazon.com, Apple Inc, Facebook and Google".

So I'd say it's pretty ambiguous.