There used to be a poster at Digital Spy, a big Sharon fan, who would talk about the damage John Yorke did to the show during his tenure, both as producer and in the overseeing job (whatever the title was) after moving on as showrunner/producer. Constant OTT plots during his run, and things like that.
I do think the fourth episode is a big factor. I wish that during the show's prolonged ratings collapse they'd made the decision to prune an episode, and with it prune the cast. Now that the ratings have increased somewhat, they likely won't do it, but they had the excuse, and they don't rely on Eastenders the way that ITV is increasingly reliant on their soaps due to many of their other programs tanking in the ratings.
I remember a story about June Brown threatening to quit if they went to 5 episodes, as they were supposedly thinking of doing 10-15 years ago.