Hi
i just returned from my holiday with the GH4 and the 14-140mm MK2, and i noticed the same issue, very intensly.
look at one of the more extreme videos here:
Image Stabilization : On, Mode 1
Focal Length : 140.0 mm
my GH4 was on firmware 1.0, and i dont know if that makes any changes.
i am very disappointed that most of my 4k videos now are quite unusable, since mercalli and premiere seem not be able to cure them