Thanks for asking! Both are great questions I’m happy to answer. There is a lot of history.
Background on the 256px option
For a long time, thumbnails were capped at 128px.
10 years ago, 256px size thumbnails were introduced by a GNOME 3 designer Increase the size of thumbnails to 256 (c5872652) · Commits · GNOME / GNOME Utility Library · GitLab.
5 years ago, the option to display 256px thumbnails in was introduced in nautilus as part of an experimental feature – the new grid view:(view-icon: add new zoom level (1e5eba1d) · Commits · GNOME / Files · GitLab)
That experimental grid view has served as the foundation for the new grid that has debuted in version 43 The icon view is dead, long live the icon view! – Blog about what I do
I personally figured it would be correct to follow up on the precedents and keep the 256px as a highlight feature of the new grid view.
Background on why 4 options
Going back in time again, long ago there were as many as 7 zoom levels: libnautilus-private/nautilus-icon-info.h · 3.0.0 · GNOME / Files · GitLab
Seven years ago this has been reduced to only 3: nautilus-icon-info: rework zoom levels (1968379a) · Commits · GNOME / Files · GitLab
This change was done under guidance from design recomendations:
Reduce the number of zoom levels to 3/4.
Bug 737189 – Refine list layout, reduce the number of zoom levels
Soon afterwards the 48px size has been reintroduced, going up to 4 options: general: add another zoom level (fd21c947) · Commits · GNOME / Files · GitLab
The work-in-progress designs for the future of the grid view also use only 4 different sizes.
So, we can see there has been a consistent design consensus on the number 4 in this context.
It would be ideal to have a lot of UX research for every design decision, but when such research is not available, I’d rather trust the design experts that have been working on this for quite a long time.