How much training is needed to run a 5km in the 4-minute range?

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I've been running a bit lately. Looking at the posts here, I often see people running in the 4-minute range.
I'm running in the 6-minute range, so I can't even imagine running in the 4s.
When I run along the river, I pass most people even while running in the 6-minute range,
but the 4-minute range... wow, aren't they basically at a professional level?