I have and it's just ok. It requires ISO 1600, 25 second exposure at f/1.8 with my Olympus 17mm to get a clean image. At 25 seconds you get too much motion blur of the stars. If you had a Voigtlander 12mm or 17mm f/0.95 lens you might be able to get down to 15 second exposures and ISO 800 which would be sufficient to get pretty impressive results.
With all that said, I use my GH4 for milky way shots all the time and it's still quite impressive vs. my old Canon APS-C cam. Here's my only raw milky way shot I've tried. I had auto white balance set and it went all over the place on this one. I also missed the focus point and the stars are all out of focus so the video wasn't worth posting up.
In the end of the day, a used 5D2 and an old manual 28mm f/2.8 works a lot better for this sort of thing with Magic Lantern.
Here ya go. They are decent (talking about my pics), definitely the best work, but these are taken with GH4 with Tokina 11-16mm DX II. You can easily take fantastic astro photos or timelapses with GH4, just got to know how to use it. It does take a little bit more to get a proper image with GH4 than with lets say 5DIII.
Exposure was 20 or 25 seconds, which I consider to be too long for my taste, but that was the only way for me to gather more light without destroying the image with noise. Some compromises have to be made when doing astro photography with GH4.
Awesome guys..... And wow love the above images..... I have my GH4 on order and it's just days away now.... So really want to try some galaxy timelapse stuff. Thanks again if anyone else has some nice images and or timelapse please post.