I told you it was coming.) For its 4th edition, An Introduction to Functional Grammar (known to its friends as IFG) has been re-branded as "Halliday's Introduction to Functional Grammar." Following on from the major revision by Michael Halliday and Christian Matthiessen for the 3rd Edition, IFG4 contains copious corpus samples and detailed descriptions of theory and sample analyses. IFG has now been revised four times in the last four decades and is published (officially in 2014), for the first time, by Routledge instead of (Edward) Arnold.
IFG4 is now much more of a reference grammar than the teaching grammar that appeared as IFG1 in 1985. This is probably because we now have the very useful guides by Geoff Thompson ("Introducing Functional Grammar" now in its 3rd Edition), Bloor and Bloor ("The Functional Analysis of English" also 3rd Edition), Eggins ("An Introduction to Systemic Functional Linguistics" 2nd edition), Butt et al. ("Using Functional Grammar" 3rd edition), Drogba and Humphrey, Martin, Matthiessen and Painter, and so on.
It seems to me, at first glance, that a majority of the revisions come from connecting to other research in SFL - an extended bibliography constitutes more than 20 of the approximately 100 new pages. This, and the extended index, add great value and functionality to the volume, particularly compared with IFG1, while maintaining almost the same analytical framework.
IFG remains the definitive guide to Systemic Functional Linguistics. It is not the only approach to Systemic Functional Linguistics, (Martin's "English Text" and various volumes by Fawcett offer alternative views on SFL analysis) but it is undoubtedly the most influential and the version that most newcomers to discourse analysis try to learn.