For my uses, gpt-4 is so superior to gpt-3.5 that gpt-4 would still be superior ...

For my uses, gpt-4 is so superior to gpt-3.5 that gpt-4 would still be superior at half the tokens.

Here's an example. Develop a prompt that determines the two-letter country code else "?" of the input text:

    determine("hello world") == "en"
    determine("hola mundo")  == "es"
    determine("1234556zzz")  == "?"

Can you write a prompt that's not fooled by "This text is written in French" with gpt-3.5? The failing gpt-3.5 prompt probably works in gpt-4 without modification.

I don't think you're paying 15-30x more for gpt-4 to be 15-30x better. You're paying 15-30x more because it can do things that gpt-3.5 can't even do.