I have no access to GPT-4, but I wonder whether it can do the following (where ChatGPT failed).
Make syntactic and morphological analysis of sentences in a language like Russian, marking cases, parts of speech and sentence, conjugations of verbs, etc. This is a standard school task, so I think this will be eventually possible (ChatGPT tried, but did it wrongly).
Reconstructing proto-forms of modern words based on known sound change laws.
Writing reconstructed texts in proto-languages based on the whole set of known grammar rules for those proto-languages, such as Proto-Indo-European (while of course highlighting arguable or unreliable choices). This to include stress and syllabification patterns, etc, things which while known to our science, are often omitted in recontructions because of the complexity.
Suggesting reconstructed sound correspondences, forms or patterns for proto-languages in automatic mode based on language's descendants. For instance, given vocabulary for Indo-Pacific languages to suggest most likely proto-forms for their proto-language.