What is this question asking?

いつ着くの？

I don't understand the role of the 'no' particle in this sentence. So far far I've understood it to specify the the type of something (sensei - koukou no sensei) or possession of something (neko - Akira no neko). So, does this mean that the question is asking what kind of "how soon to arrive" ? And would that make the answer, if I spelled it out,
"着くの四じです" ?

It's not the possession particle, but a sentence final particle.
Noun-related particles - Tae Kim's Japanese grammar guide

着くのは4時です, 4時に着きます or more simply just 4時です.

Toritoribe said:

It's not the possession particle, but a sentence final particle.
Noun-related particles - Tae Kim's Japanese grammar guide

着くのは4時です, 4時に着きます or more simply just 4時です.

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Thank you. That was much simpler than I had thought.