Usually, yes: many DSLR zooms still have a mechanical link between the focus ring and the focusing mechanism, so they can be manually focused with a simple mechanical adapter even if autofocus and electronic aperture control are lost. However, some lenses are focus-by-wire or otherwise electronically dependent, and those will not focus properly without power.

So the answer is lens-specific, not universal.

For the Sigma 18–200mm mentioned in the replies, adapting it mechanically is possible, but one user noted it isn’t especially strong optically wide open and can seem relatively dim compared with faster prime lenses. That’s normal for slow superzooms.

Also, since you’re using a GH4, it may make more sense to look at a native Micro Four Thirds superzoom first. A community reply pointed to the Olympus 12–200mm f/3.5–6.3 as a native option that covers a very broad range without the uncertainty of adapting electronic DSLR lenses.

If you adapt a non-native electronic zoom, verify beforehand whether its focus ring is mechanically coupled and whether aperture control matters for your adapter setup.