Are there any cheap landscape lenses for a GH4?

It all depends on what you mean by "not too wide". A "normal" lens (c.50mm) is reckoned to be one which gives a similar field of vision to the human eye, so I will go with that.

The Panasonic 25mm f1.7 lens fits that bill because with m4/3 you double the lens focal length to get the equivalent in 35mm ("full frame") terms. That's because the diagonal of the m4/3 sensor is 1/2 the diagonal of an FF sensor. You should be able to get that lens for the sort of amount you mentioned.

The older Panasonic 20mm f1.7 is also available now at a good price, especially used, and is an m4/3 classic due to its small size, sharpness and contrasty rendering. It can be good for landscapes. Its weakness is slightly slower autofocus compared to the more recent 25mm f1.7 - and complete lack of continuous AF capability. It is still usable for video if you don't mind using single AF or manual focusing.

Both of the above are prime lenses, meaning they have a fixed focal length. They will give you the best value in terms of light gathering ability - zoom lenses are nearly always more limited in that regard, with a typical kit zoom starting at f3.5 or f4 as its widest aperture at the minimum focal length. If you're willing to forgo that for their greater flexibility, there are several zooms you could acquire quite cheaply in the 14-45mm zoom range.  The Panasonic 14-45mm f3.5-f5.6. is one example that should be available used for the money.

Bear in mind to that the GH4 uses a considerable crop in 4K video, so you will need a wider lens in 4K to get the same field of view as a still photograph.

To sum up, one of the prime lenses will give you the best value in terms of image quality, particularly in low light. Against that, you will be restricted to a single focal length and will have to "zoom with your feet". A kit zoom will give you much more flexibility for framing and should give decent results in good light.