my playback device is the iMac I am making the keynote on, and it's resolution is 4096 x 2304.
As your playback device is the iMac, create a Keynote presentation with a slide size of 4096 x 2304 matching the iMacs Retina display resolution:
Inspector > Document > Document: select Custom from the Slide Size drop down menu and enter 4096 in width and 2304 in height.
You will then have a spectacularly impressive high def resolution presentation.
There is no need to create a Quicktime export at all.
What if I was displaying the keynote on a 5k iMac screen? 3840 would look a bit blurry.
Again, you would not need Quicktime files to play the presentation, you would playback the presentation in Keynote, with a slide size set at 5120 x 2880 pixels to match the 5K display resolution.
Why offer custom export settings if they are locked?
Quicktime export from Keynote can create video files to many international video standards, it also provides custom sizes, for example 425 x 425 pixels for inclusion in a web page. However, Quicktime export from Keynote, does not encode greater than 3840 x 21604K UHD video.
What does the software care what resolution I export at. If i have the HD space and the CPU why am I locked out of that setting?
To export ultra high def video from Quicktime, ultra high def export CODECs must be available to the QuickTime export module.
At this time, Keynote only has 3840 x 2160 UHD CODECS available. Keynote is primarily a tool to create presentations using the Apple Keynote filetype. Keynote is not a specialist Quicktime video creation tool.
All of the other video tools and software I use have no problem exporting at larger resolutions using quicktime.
The export module in Keynote is set up differently to other applications, ultra high def CODECs above 3840 x 2160are not available to Keynote.
If creating ultra high def Quicktime video files is important to you, then you should be using dedicated video creation tools such as Apple Motion or Adobe After Effects.
What if I wanted to export from my 5k iMac? I would be stuck with a keynote exported at almost half the resolution my display supports.
For the average Keynote user, exporting to UHD 4K video is more than acceptable.
Why do you want to encode 5K video?
Its not practical for the average Keynote user to encode video files at: 5120 x 2880 pixels resulting in 3 GB per minute file sizes.
A 30 minute presentation has a minimum file size of 90 GB, how many of those file sizes do you have storage capacity for?
Extra resources would be required to take care of that data rate;
16 GB RAM minimum required
external high performance RAID Array drives to store rendered files
external back up drives
external archive storage drives
If there was a genuine need to do this, you would be using more appropriate applications anyway; Apple Motion or Adobe After Effects not Keynote, using Apple Compressor not Quicktime Export, for all the reasons previously given above.