Thinking of upgrading from a Nikon D5100 to a Panasonic GH4?

I have a Nikon lens already, the Tamron 17-50mm f/2.8. Will it be compatible as is or will I need to purchase a Metabones EF to M43?

I'm using the camera strictly for video, will there be a noticeable difference in video quality from this upgrade?

The D5100 was great to learn from. But I'm just not getting that "professional" look. I shoot music videos and market through youtube so video quality is essential.

As soon as you start trying to cobble together bits of incompatible kit using adapters, the first thing to suffer is image/video quality.
The next thing you realise is that you lose auto-focus, auto-aperture and if neither your lens nor your adapter has manual aperture control you can only use it at max aperture. With other adapters you lose the ability to focus at infinity.

So forget keeping your Nikon fit Tamron lens and buy one with a micro 4/3 mount.

I can't comment on the relative merits of the 2 cameras you quote except to say that the "professional" look you seek is 90-95% down to the photographer and his/her use of light with maybe 5-10% down to the camera

If you can't get THE LOOK with a D5100, you won't either with whatever camera you get out there. Skill is essential. It will dictate what tool you prefer to use. Judging from those two cameras you're mentioning, you're not there yet.

Here's YouTube video comparing the D7100 to the GH4. Since the D7100 is a better and more expensive camera than the D5100, and the fact that the D7100 can't hold up to the quality of the GH4, then the D5100 should be outperformed by the GH4.

Currently Canon and Nikon have fallen way behind Sony and Panasonic in terms of their video quality and their video capabilities.

Let me add one more voice here. If you are mainly in this for videos - get a video camera! Any still digital camera is a compromise for videos.