PHP + iconv - Transform UTF-4 string?

Question

I'm writing an E-Mail parser. I noticed that I received some emails that state their charset is UTF-4.

However, when trying to convert these with iconv to UTF-8 it fails.

Now my question is: I've never ever heard of UTF-4. Is this even a valid charset? And if not - can I just treat it as UTF-8?

Here is part of the mail header:

["mime-version"] => string(3) "1.0"
["content-type"] => string(24) "text/html; charset=utf-4"