Why isn't this considered absurd? What a joke!
In addition, they have Evernote apps for almost every major platform [1] (Mac, Windows, browser plugins for Chrome, Firefox, and Safari, plus iPhone, iPad, Android, Blackberry, WP7, and webOS). If you're not on any of those platforms, or those apps aren't installed on the machine you're using, you can ALSO use their web interface.
All of that is supported by a ridiculous server infrastructure. They jumped from 4M to 10M users in 10 months [2, 3], and that was 2011. They're now at 34M users. [4]
Evernote users aren't just tweeting back-and-forth, either. They're saving PDFs, uploading scanned notes from class, searching through OCRed image results, clipping web pages, etc. That all requires tons of storage, as well as the infrastructure to retrieve and back up that storage as quickly and reliably as possible.
Soooooo yeah they're a pretty big deal. 4 engineers for a native app is an understatement--between product managers, designers, front-end work, back-end work, and ops team, you're probably looking at a 12-18 person team per serious app. Possibly more, if it's popular (like iOS or the web interface).
EDIT: estimated 20-30 person teams, but that's a bit much. Probably less than that
[1] http://evernote.com/evernote
[2] http://www.intomobile.com/2011/06/09/evernote-hits-10-millio...
[3] http://mashable.com/2010/08/17/evernote-surpasses-4-million-...
[4] http://techcrunch.com/2012/06/19/evernote-by-the-numbers-34m...
Your post is the business equivalent of a developer saying "I could create a stackoverflow clone in a weekend."
1) Infrastructure 2) Building out a kick-ass enterprise sales force 3) Global expansion 4) Acquisitions 5) Liquidating existing investors