Testing with QSTranscode 1016, I noticed that converting a 4k video to 4k it fails immediately when using Lookahead bitrate mode. Screenshot: http://s1.directupload.net/images/130920/yo9dirfk.png
Another 4k video (4096x2304) appcrash: http://s7.directupload.net/images/130920/3wk32xz2.png
VBR mode works fine. Also 4k to 1080p no issue with Lookahead.
I forgot to mention I'm using Haswell i5. Ivy Bridge and Sandy Bridge does not support 4k encoding in hardware (only via Software SDK), so that's not the problem.