When it comes to search google is king when it comes to total traffic yahoo has been on top for years and years. Although google is getting real close.
I think Google took too much share, such that even 2nd and 3rd are already quite redundant, anything after this would not arouse much interest.
Ask.com is definitely the 4th. For me, AOL brings in about the same traffic as ASK, but AOL uses Google search.
He is asking about "SEARCH ENGINE", not the entire web. According to compete.com, Google rules the search engine world, Ask is the fourth. Here is the image data: Alexa doesn't know anything about who rules the search engine, they only count traffic.