Offtopic: The 4th big is twitter? I thought it was Microsoft. I also thought that there are at least a dozen companies above twitter as far big goes in any possible sense (revenue, salaries, dev happiness, etc.).
I don't know, maybe it was Microsoft. I haven't put much thought about what is "the big four". To me, they all fall into the same bucket: big, computers, mostly US.
1. Apple 2. Alphabet 3. Microsoft 4. Berkshire Hathaway 5. Exxon Mobile 6. Amazon 7. Facebook
If we're going by impact on tech, opportunities for software development, etc., it could potentially be different.