It'd be entirely up to the distributor of the data, so perhaps the answer to your question is "all of the above".
For example, (1) our command line tools use URL-like paths which implies "use this hostname" (to copy-paste into terminal), (2) we have some in-browser visualisations like http://splore.noms.io/?db=http://demo.noms.io/cli-tour which implies more of a "click here" type UI.
Keep in mind (if this wasn't clear) that the chunks are only probabilistically 4K: https://github.com/attic-labs/noms/blob/master/go/types/roll.... I.e. the thing that's "fixed" here is the chunk size we're aiming for. The chunks themselves could be of any size.
In any case, that's a good question - we might want to do something about that down the line. But, if we did change that constant, the structure of the trees will change, and all[1] the hashes will change.
For example, (1) our command line tools use URL-like paths which implies "use this hostname" (to copy-paste into terminal), (2) we have some in-browser visualisations like http://splore.noms.io/?db=http://demo.noms.io/cli-tour which implies more of a "click here" type UI.