$4bn for an easily replaceable service that is little more than Microsoft Paint duct taped to a disposable camera?
Oh please!
Sure you can recreate the app pretty easily which, you can also do for most services (this does not mean scaling to the users they serve) but that is not the value of the service.
It's also why 'hackers' typically don't make great investors see: https://news.ycombinator.com/item?id=8863