You can however define inputs that are not the whole of nixpkgs. You would use something like this and you would pin it to a very exact version and hash of a package:

https://nixos.org/guides/nix-pills/nixpkgs-overriding-packag...

The goal of a downstream Linux distribution is never to reproduce whatever builds you run on your own machine as an upstream developer. It's to produce a collection of installable software that meets various constraints and goals, like cohesion (can all be installed and managed uniformly), minimal size, easy/manageable security updates, integration (compatibility and so on). That can involve things like building the software against particular library versions mandated by downstream needs or even patching it. Some distros try hard to avoid patching upstream and some don't, and in all distros there may be cases where other priorities take precedence over the value of leaving upstream untouched.

In the case of Nixpkgs and Python, the community wants to maintain a collection of Python libraries that are all interoperable, and Python doesn't support vendorization well enough to allow multiple versions of the same library in a single Python process, which is one reason for preferring singular versions of most Python libraries in Nixpkgs. The other factor is likely just reducing the maintenance across Nixpkgs by maintaining as few redundant versions within the tree as possible.

If you want to control/determine the entire runtime your end users use, you have to do the packaging work required to ship them that runtime with some tooling that's capable of the reproducibility you desire. Python doesn't have one a reproducible package manager, so your options are basically creating your own Nix package (probably as a flake.nix in your repo), Docker, and Flatpak.

That said, it's perfectly possibly to include multiple minor releases of Django 4 in a single snapshot of the Nixpkgs tree and maybe that should be done. Have you talked with the maintainers of your downstream package of Nixpkgs to let them know Django breaks things on minor releases, and so using different versions of Django 4 interchangeably is not tested or supported in your application?

You can pin the commit of nixpkgs. Why should reproducibility hold when nixpkgs changes?