S3 was such a rebound from the soap opera nature of S2, but the ending essentially killed the series IMO. There seemed to be a pretty clear path forward for the show, an overarching plot that could have lasted a season or two. But S3's end essentially forced the show to become stuck in a loop of rehashed plots and side stories that were much more interesting the first time around. S4 wasn't horrible...but the downward spiral is pretty clear now. Granted I did like how S4 ended though.
It's pretty clear the show runner wasn't prepared for what happened, and I can't really blame him tbh.