Which is correct? log in, log on, log into, log onto [duplicate]

I've seen different questions related to the same verb, but those questions implicate an imperative form (For example when you tell somebody to log in/on) which I think may vary the condition in which the verb is used.

1. He uses another program to log in to the system.

2. He uses another program to log into the system.

3. He uses another program to log on to the system.

4. He uses another program to log onto the system.

I think number 2 is correct but I'll wait for your suggestions.

Solution 1:

For my money, log on to a system or log in to a system are interchangeable, and depend on the metaphor you are using (see comment on your post). I suppose there is a small bit of connotation that "log on" implies use, and "log in" implies access or a specific user.

Not to be confused with "login" - a noun describing a combination of username/password.

I'd pick 1) because the program is accessing a system (in vs. on), and "log in" is the action, whereas "to" begins the prepositional phrase.

So, he uses another program to log in to the system.