I realize I may be in the minority, but I love series 4, so much so, actually, that I'm tempted to call it my favourite series (but then I remember some of the too-brilliant-for-words earlier episodes and just can't make up my mind).
The truth is, if you are expecting AbFab to pick up exactly from where it left, you'll be better off not getting your hands on series 4, because half a decade has passed and the world just isn't the same anymore. What was cool, fashionable and modern before, is now old and dated, at least for the likes of Edina who only live to keep up with the latest trends. There definitely is a huge visual contrast to what the show used to look like, but don't let that get on the way. Edina and Patsy are still at it, crazier (and also more desperate) than ever; experimenting with parralox, dieting with detox, popping pills, going after men, all the while trying to avoid getting old.
All I can say is give it a go and see what you think. I think it's absolutely fabulous.