NOTE: I am stupid and misread your diagram as komoko (3-4) instead of hoshi (4-4) in the corner. The following applies to komoku only! I was going to delete this answer, but then decided to leave it up, because the question would probably be interesting for komoku, too.
W4 is actually joseki, but it is very, very rarely played. I have never witnessed it in high level matches, and I would not at all be surprised if dan players do not know it (the opposite, rather).
I learned it in a book by Yilun Yang (Whole Board Thinking in Joseki). Basically, it is a move that moves white out very steadily (i.e. slowly). Sadly, I do not have the book with me for an extended period of time, but maybe someone else can jump in an add a more detailed explanation of the move.
Either way you don't have to worry about it, really - if your opponent plays W4, deal with it as you would anyway (remember that a non-joseki move can be good sometimes, and also that a joseki move can be bad sometimes, too). If you feel that W4 is a good move on a certain board, don't be afraid to try and play it. You'll see how it turns out!