Yeah Greg - I thought the same actually - after delivering 2k for so long (2012/Apes @4k for some shots) -
But Tim is exactly right - Even Avengers (which was shown IMAX) - I delivered final comps at 1920x1080 - freaked me out as well as I'm super anal on pixel precision - but it looked perfect in IMAX. Everything I've been doing lately is all closer to 1080 or exactly 1080.
Elements are typically in the 2k range, but since I design in Illustrator - I always retain the option of continuous rasterization. I'll use that for times when there's a gigantic screen and you're seeing just a corner of it - like a few scenes looking over James Franco's shoulder in Planet of the Apes - or the Mess Hall Scoreboard when you're focused on Ender.
The final thing that helps is to remember when we comp we're going for that 'filmed in camera' look as opposed to a 'graphic' or 'video' tack sharp look - so, to the graphics, I'm always adding an initial fast blur (usually around .5), then motion blur, then Bokeh/DOF blur (frischluft!), then rgb separation or chromatic aberrations, a super subtle glow, and then film grain on top of it all to match the plate footage. The final look is as forgiving to scaling as film is and you can get away with a lot.
Hope that helps answer your question!
// jayse