Usage of "has" vs "have" nowadays

Question

I'm here again with misunderstanding of some grammar rules. So there is sentence where it's said " 4 days has never felt so long". why a song writer used "has" instead of "have", when I asked question bout "he do" vs "he does" I got what's the point of it, but here, I can't understand. There is no rhyme, there is nothing that can be cause of using "has" instead of "have"

Here is the lyrics: Baby, I don't feel so good Six words you never understood I'll never let you go Five words you'll never say (aw) I laugh alone like nothing's wrong Four days has never felt so long If three's a crowd and two was us One slipped away

Answer 1

Whether you use the plural or the singular depends entirely on how you are interpreting the noun.

For instance, some people say this:

Your twenty minutes starts now.

This means that there is a single period of time (singular) that totals twenty minutes.

So, after one minute has passed:

You now have nineteen minutes.
→ Your twenty minutes is still not up.

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However, other people say this:

Your twenty minutes start now.

This means that there are twenty intervals of time (plural), consisting of a minute each. After each minute passes, that minute is gone.

So, after one minute has passed:

You now have nineteen minutes.
→ Your twenty minutes are not all gone.

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The song lyric in the question can be interpreted in the same way. The verb used depends on how the four days are viewed.

Four days has never felt so long.
→ A single period of time that totals four days has never felt so long.

Or:

Four days have never felt so long.
→ Four individual days, one after the other, have never felt so long.

Answer 2

It might seem a bit confusing because "days" is plural, which makes "have" seem like the appropriate choice. However, "4 days" is a single period of time, so "has" agrees with the subject in that case.