You're only showing us a bit, a lot depends on how much of the walls are tiled and how well they are attached to the walls and the state of the surface of wall when you've finished.
In what bit we can see some of the surface has come away from the wall in one place and there's some tile cement attached in another.
You could try removing all the tiles. Some will probably practically fall off (like the first one above the missing tiles) others you might have to fight to get off. in any event when they are off, there will be a lot of preparation repairing the plaster required, before it is fit for painting. As long as the wall behind the tiles is sound, after removing any residual tile cement, you should be able to skim over the imperfections with some plaster and a straight edge like a long ruler to scrape across it, if you've not the proper tools. As long as each end of the ruler is on a flat surface. It does take a bit of skill, but you can practice on one small area until you get the hang of it. However, you won't get it as good looking as having the walls professionally re-skimmed.
Paint will show up the slightest imperfections.
I can't see an "easy fix" other than replacing the missing tiles.
That might not be easy either. Such 4" tiles of that age are usually now made of "unobtainium."