not to mention and let alone

Question

"Let alone" is often treated as a synonym of "not to mention." But are they interchangeable? Are the following sentences correct? If only #1 and #2 are correct, what's their difference?

1. John can speak Classical Greek, Latin, French, and Japanese, not to mention English.

2. John can speak English, not to mention Classical Greek, Latin, French, and Japanese.

3. John can speak English, let alone Classical Greek, Latin, French, and Japanese.

4. John can speak Classical Greek, Latin, French, and Japanese, let alone English. (This one is definitely wrong; added here for contrastive purposes.)

I'd appreciate your help.

Answer 1

not to mention is used in situations where an already impressive fact is augmented, in (feigned) afterthought, with a yet more impressive fact.

The house has two tennis courts, a five-car garage, and an Olympic-size swimming pool, not to mention access to a private beach.

The house has two tennis courts, not to mention a five-car garage, an Olympic-sized swimming pool, and access to a private beach.

She has volunteered at the library and at a local soup kitchen serving the homeless, not to mention the hundreds of hours she has devoted to the town as chair of the recycling committee for the last eight years.

As discussed in a similar question you asked recently, let alone is used in situations where the lesser of two items (in a given context) has already met or exceeded a threshold, so that there is no need to invoke the greater one.

His erratic driving had given the police probable cause to search the vehicle, let alone the fact that his passenger was bound and gagged.

We have no need for another car, let alone no place to park it.