I tried this:

SCENE:

George to Kramer: "What's the cat's name?"
Kramer: "Elaine."
George, impersonating Kramer, on the phone with Jerry: "Hey Jerry, what's wrong with Carroll? I can hear her screeching heavily. Is she all right?"
Newman, impersonating Jerry, Kramer's good friend and neighbor: "Carroll fine, bro. Carroll just fine. What are you up to?"
George hangs up the phone and says to Kramer in normal voice: "Jerry is dead."

How did George figure out Jerry was dead?

and GPT4 responded with:

Run 1:
George figured out that Jerry was dead because Newman was impersonating Jerry on the phone, which was unusual and suspicious. The way Newman responded with "Carroll fine, bro. Carroll just fine" was not consistent with Jerry's typical manner of speaking, and this alerted George that something was off. Additionally, Newman referred to George as "bro," which is not a term that Jerry would typically use with George.

Since Newman was impersonating Jerry and George knew that Newman was Kramer's neighbor, George may have assumed that something serious had happened to Jerry, like his death. It's important to note that this is an assumption George made based on the information he had at hand, and it might not be entirely accurate.

Run 2:
George figured out that something was wrong, possibly that Jerry was dead, because of a few clues:

The voice on the other end of the phone didn't sound like Jerry. Instead, it was Newman impersonating Jerry, and George was able to detect that it wasn't Jerry's voice. George and Jerry are good friends, so he would likely be able to tell if someone else was pretending to be Jerry.

The response "Carroll fine, bro. Carroll just fine" was not in Jerry's typical speaking style. Jerry doesn't usually use the word "bro" and his speech is generally more articulate. This change in language indicated that something was off.

The fact that Newman was impersonating Jerry suggests that something has happened to Jerry. If Jerry was alive and well, there would be no reason for Newman to pretend to be him on the phone.

While George's conclusion that "Jerry is dead" might be a bit extreme and not necessarily accurate, these clues led him to believe that something was very wrong with Jerry, and his immediate assumption was that Jerry must be dead.

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The GPT4 is obviously confused by its Seinfeld training data set. I didn't make any reference to Seinfeld. I also changed the dog to a cat and used 'screeching' instead, but it doesn't catch the switcheroo.

Even when elaborating: "the scene had nothing to do with Seinfeld the TV show. Can you reevaluate the question?"

I got:

Apologies for the confusion. Based on the scene provided, George figured out that something was wrong, possibly that Jerry was dead, due to the following reasons:

George called Jerry to ask about Carroll screeching heavily, expecting to get some information about the situation. However, the response "Carroll fine, bro. Carroll just fine" was not very informative and seemed dismissive, which raised suspicion.

The voice on the other end of the phone didn't quite sound like Jerry. George noticed that the voice was different, which led him to question who was actually speaking.

The change in speaking style, specifically the use of "bro," was uncharacteristic of how Jerry usually speaks. This indicated that something was off.

While it is not certain that Jerry is dead based on this information, the inconsistencies and suspicious behavior during the phone call led George to jump to that conclusion.