How do search engines index multilingual content?

I am building a website with identical content in four different
languages. On a first visit, the search engine determines the language
of the content by the IP address of the visitor. What the user sees is
content in only one language at a time. He or she can then switch to
another language and set this as the preferred language, but again he
or she sees content in only this one other language.

The question now is: How do I get search engines to index ALL of the
content, in all languages?

Should I include the non-displayed content in DIVs with display set to
"none" (like we used to include complete websites in the noframes tag)?
Or do search engines ignore invisible DIVs?

Or can I somehow detect that a search engine is visiting and deliver a
page with the complete content of all four languages in it?

Or do I have to rely on the search engine following the local links to
the pages in the other languages? This might be a problem, because the
varying content is always displayed on the same page, so the URI stays
the same, and only one parameter changes, thus:
"content.php?language=oneoffourlanguages". In fact it might even be
impossible, because I do not want to transfer the language information
through the URL via GET, but want to send it through a form via POST.
So the URI is exactly the same for all languages (at least in the
version I am aiming at).