Sony Vegas 12 and 13 can edit at 4K resolution and handles the footage from our various cameras very well. It did not like using MP4 files created by the Panasonic Gh4 camera but was happy using the MOV files created by this camera. As a Sony program it also has excellent support for Sony XAVC files, as long as you are using the latest version of the program.
We normally like to monitor 4K files on a proper output monitor via some hardware like the Blackmagic UltraStudio 4K. Although Vegas supports the Blackmagic hardware it will not output 4K from them currently. This will require an update to Vegas, and we do not know when this will be coming.
Apart from that editing 4K worked well and Vegas could manage 1-2 layers of footage on an up to date system. It also has a range of output options for 4K files.
As with all the programs you should aim to use the latest version, Vegas Pro 13, as they are adding new format support and fixes in the newest version of the software only.