According to a report from The Korea Times, Samsung (005930:KS) is planning to unveil a new Galaxy S4 in February, and will begin selling it globally in March. The report cites a rep from a Samsung partner:
Samsung wants to keep its one-year product schedule and the Galaxy S4 will be the first to match that strategy. The S4 will see some external changes but retain its popular rectangular shape with rounded corner concept.
This seems just a little bit soon to me, given that the S3 only launched in May.