I'm talking about a trip to South America. I say:

Four days aren't enough. Or

Four days isn't enough.

Please proofread! Thank you!
1 2
Both are used. I'd say "isn't". I think of it as one sequence of four days, so my brain tells me it's singular. Emotion: smile

I agree with CJ. However, “isn’t” is more informal. You can also say “A four-day trip just isn’t enough.”
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Word warrior23However, “isn’t” is more informal.
I don't think it is. It's just a different way of looking at it.
Look at it as

(A period of) four days isn't enough.
Huh. Saying "isn't" would never have occurred to me! Odd. Emotion: smile Would "aren't" sound wrong to you?
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Leah SSaying "isn't" would never have occurred to me!
How about:

$25 is a lot to pay for a hamburger.
Three miles isn't far to walk.

Leah SWould "aren't" sound wrong to you?
Not in the original sentence. It would in my two sentences above.
"Are" would be ridiculous in those two, I agree. Maybe it's to do with the complete distinction of four individual days in my mind. Emotion: thinking I have no idea. What would you say about two weeks being enough? Or two cars?

But I think if I were discussing a number without a noun, I would say "is". All these examples are confusing my initial reactions, though.

eg. "How many buses will you need for the school trip? Five?"
"No, four is enough."

Never thought about it, so no rationale. Thoughts?
fivejedjon Word warrior23However, “isn’t” is more informal.
Well, I wouldn't write that way in a formal letter. Emotion: wink
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