The only "change" in DO residencies is that in EM (and many, many others) it's no longer a one year rotating internship and then a three year residency -- the internship year is part of the EM residency and is EM-focused. (Though some programs maintain more of the feel of traditional rotating year than others). So tkim's quote is out of date wrt osteopathic EM -- this changed pretty recently.
The 3-vs-4 debate is a tough one, with (it seems to me at this point) good arguments on both sides. I've applied to and am interviewing at about equal numbers of each; although the 3 years may predominate overall, there's some geographical variation. My general sense, having interviewed at some of each so far, is that the 4-year programs claim to be the better route to academia; if you have any inclinations that way and do a 3-year, you're "required" to do a fellowship to be taken seriously.