Yes, still cameras can do things 4k video can't. I imagine that future cameras will go in the direction of the Canon 1D C, which has still photography and 4k video in the same body.

From a photography standpoint, the advantages of shooting stills instead of frame-captured video include:

• Higher resolution. 4k video is around 8 megapixels, compared to the 16-36 megapixels we find in standard DSLRs.
• Faster shutter speeds. For video, the shutter speed is normally fixed to twice the framerate, so a 30 fps video would be shot with a 1/60s shutter speed. The effect is that movement will appear smooth in the video, but individual frames will be slightly blurry, which you may not want in a photo. If you increase the shutter speed enough to freeze the motion in individual frames, movement in the video becomes strangely staccato, sort of like watching movement under a strobe light. So even if you use 4k video for both video and photography, you will often need to decide ahead of time whether the recording is intended for video or stills.
• Slower shutter speeds. For a 30 fps movie (for example) you can't possibly use shutter speeds slower than 1/30s. For photography, you will often want longer than that, e.g. for deliberate motion blur, light painting, low light, astrophotography, etc.
• Simpler exposure. Photographers as well as videomakers often use aperture for creative purposes, e.g. to get a shallow depth of field. In still photography, you can simply adjust the shutter speed to get the right exposure. For video, on the other hand, the shutter speed is a function of the video framerate, so in bright light you have to resort to ND filters.

4k video can still be useful for photography. The advantages of video over stills include:

• Higher frame rates. Even the fastest professional DSLRs top out at 10-12 fps, while video routinely gives 30 or 60 fps. Although you trade off some image quality, the quality may be good enough for the purpose, and these kinds of speeds are not available in normal cameras.
• "Micro expressions". A wedding photographer who did an early review of the Canon 1D C found 4k video useful for capturing what he calls "micro expressions", tiny changes in facial expressions that can make two photos taken seconds apart look very different. With 4k video, he could simply record a longer segment and pull out the frame where the bride and groom look their best.

In conclusion, there's no reason for cameras to disappear, and photography and video/filmmaking are still very different processes.

But 4k video integrated in cameras can be an additional photographic tool that will be useful in some circumstances.