A quick wiki visit says the term open source existed before the OSI. But that's not really important.
I just wanted to point out that UE4 does a better job than many other engine (and software!) vendors, since they give you a peek at the source and I think this should be praised, rather than scolded for not being totally free.
Oh, you know, the OSI[0], the folks that coined the term in the first place. This is a textbook case of "openwashing"[1].
[0] http://opensource.org/osd [1] http://michellethorne.cc/2009/03/openwashing/