I post-produced my timelapse entirely in 4K, which was a first for me. Shots out of a DSLR are higher resolution than 4K anyway, so it's great to be able to use that resolution. Unfortunately I don't have a 4K monitor, so I've been unable to watch my own film in the highest possible quality.
Suitable intermediate compression formats for 4K is a bit thin on the ground yet, so I used 16-bit TIFF files (at full camera resolution, 18 megapixel) between Lightroom and After Effects, and 12-bit 4K DPX files between After Effects and Premiere. Both these formats are uncompressed, and added up to over 500GB. I was unable to make a final master sequence using DPX, since Adobe Media Encoder can't add audio to an image sequence when compressing into H.264, so I had to export the files for Vimeo/Youtube upload directly from Premiere.
Youtube now allows 4K uploads, so here it is. Vimeo doesn't yet, but I'm sure they will soon enough.