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.
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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.
Leah SSaying "isn't" would never have occurred to me!How about:
Leah SWould "aren't" sound wrong to you?Not in the original sentence. It would in my two sentences above.
fivejedjon Word warrior23However, “isn’t” is more informal.Well, I wouldn't write that way in a formal letter.
Word warrior23 fivejedjon Word warrior23However, “isn’t” is more informal.Well, I wouldn't write that way in a formal letter.I would. It's natural and correct.
Word warrior23wouldn't write that way in a formal letterBut you could; it would be fine in a formal letter as "Four days is not enough".