I have been hoping someone would eventually write a solid utility to use old tablets as touch screen displays for other machines but I think the apple ecosystem has prevented any good solutions from being banged out.

You can run Python reasonably well with pythonista or Pyto (I like pythonista, but Pyto supports some PyPI libraries). Apple's Swift playgrounds is pretty decent for Swift. They're all only up to a point, but you can do plenty of actually interesting stuff with them. I use them on my current iPad (and run the Python scripts on my phone).

But 4th gen is old, so it's quite possible none of that works. Maybe web stuff with something like Textastic if you pay for shared hosting somewhere, or a low end VPS isn't crazy expensive and lets you run code. If it's consistent power that's your concern, raspberry pis can be paired with one of those portable USB batteries if it can be charged and send charge at the same time.

If those options are still too expensive, really no clue. It's hard with no money at all.

Google’s project idx might work for you, https://idx.dev/ it can pull from GitHub and run an instance of vs code and a terminal in the browser. It’s not the fastest, but I’ve been using it to do some dev work with my iPad on an Astro project I have

There are no “real” alternative OS available for apple devices. You can get some IDE “lite” apps like Visual Code, Python Editor, etc. but these a meant as companions to a PC IDE not meant for compiling, linking, testing, etc. You might be able to trade the old iPad for an old laptop you can put a flavor of Linux on.

You don't say what type of programming you do, or if you're just learning or developing professionally, but maybe something fully online would work? For example repl.it, stackblitz, or Visual Studio Code for the Web.

I second the suggestion for getting a Bluetooth keyboard for the iPad.

Looks like it's a generation too old for installing linux on it directly. Too bad.

I wonder if the iSH app allows installing a compiler.

Maybe add a bluetooth keyboard for programming?

You absolutely want a keyboard when programming, but you can get one that works with the ipad. As someone else suggested the software environment is probably more the limiting factor.

I don't use Visual Studio Code, so please don't take this as endorsement, but this article reminded me of your post:

Coding on iPad using VSCode, Caddy, and code-server