Depends on what you're playing, and what you hope to get out of it.
Purely from a power POV, I'm guessing that a 1070 (I run a 1080) can do High/Very High settings at 1080p at 60fps in most modern games. If you jump to 4K and keep those settings, I reckon you might only see 30fps which for me wouldn't be acceptable and misses the point of PC gaming really. To get the 60fps you'll prob have to drop down to Med settings, so things won't look as pretty in terms of effects. You will, of course, have more pixels so it'll look sharper. As games get more fancy, you'll likely be dropping that setting down even further to maintain 4K/60, which means that to keep things looking nice you'll need to upgrade your gfx card earlier than expected. 1440p is a good middle ground, so I'd say that's worth looking into.
Personally I run 1080p/60 and keep everything on Ultra. I have no desire for more pixels as I'd rather see the fancy shadows, lighting and textures that the top option brings than more pixels right now. That's not to say I wouldn't like 4K but I'm not prepared to sacrifice framerate or prettyness right now, and nor am I interested in spending another £400 on SLi to stay where I am. I've looked at buying a new monitor in recent months, and if I was going to I'd be looking a G-SYNC monitor first and foremost. Sure, it's be 1080p still, but oh dear god is the refresh rates on those something to behold! For me they look much more visually impressive than anything in 4K. Sorry to throw another option onto the table I want quality over screen size these days.