I hadn't heard of "big 4" before. I thought it might refer to tech companies, like Google, Apple, Microsoft, Oracle, etc. but couldn't figure out which 4. http://www.techrepublic.com/blog/tech-sanity-check/schmidt-c... says "Google, Apple, Amazon, and Facebook", are the big 4 category of 'platform companies' while http://battellemedia.com/archives/2011/12/the-internet-big-f... uses the phrase the big 5, which also includes Microsoft. Then again, the speaker in https://www.youtube.com/watch?v=YJZCUhxNCv8 comments about working at the "Amazon, Facebook, Google and Microsoft - sometimes called the Big 4".
This suggests that the "big 4" is more a concept then an actual definition.
On the other hand, further search gives another possibility. The big 4 can refer to the audit firms of Deloitte, PwC, EY and KPMG ( https://en.wikipedia.org/wiki/Big_Four_%28audit_firms%29 ). It was "Big Eight", then "Big Six" and "Big Five", before ending up with 4. At http://www.narrowingthegaap.com/recruiting/how-to-get-hired-... you see "How To Get Hired By the Big 4: The Hiring Process (Part 1)" which starts "The Big 4 hire only the best and the brightest. This is only a partial truth at best. I know this because I have met some really dumb people working for the Big 4. So no, to get hired by a Big 4 you don’t need to have a 4.0 GPA with a double major in ..".
So does the github project mean the audit companies, or the tech companies? I think the tech companies are more likely to offer $200K jobs for programmers, but have little knowledge of the audit world.
I'll also hypothesize that the term "big 4" came from the audit world and was reused in the tech world by analogy.