The very first Diana Wynne Jones book I read was The Crown of Dalemark. Which happened to be the fourth book in a quartet (although, to be fair, the books were more in the same universe than directly following each other). I got lost on a few points (Hildy, mostly, and Ynen and all their backstory, that confused the crap outta me), but I followed the plot and enjoyed it too - I still like to reread it, and I'll admit I've never made it through Cart & Cwidder or the Spellcoats because I want them to have more Mitt (Moril and the Undying? Whatevs bro).