Season 4 isn't the worst. That award goes to season 1. The writers were trying to find their footing, the actors were adjusting to their roles, and there's this unpleasant feeling of lameness and cheesiness that continues a little ways into season 2. Season 1 also has the misfortune of "I Robot, You Jane," the 'big bad Internet' episode, which I have been itching to break down in a blog post for years now. Oh, and there's also the mid-90s hair. Can't forget about that. 🙂
Season 4 is actually pretty good. It has a few of my favorites, like "Living Conditions" and "Something Blue." There was the "Harsh Light of Day" and the 2-part Faith revenge episode. Angel facing off with Riley was awesome. And "Hush." It has "Hush!" One of the best episodes in the series! Adam seemed like a big doofus at first, but he was scheming and calculating, indestructible, and for crying out loud he killed a child. You can't get much more uncomfortable than that. The first season of Angel started up during "Buffy" season 4 time and the "Buffy" and "Angel" shows crossed into each other several times in unique ways.
Now, it did get irritating when it took Buffy several episodes to get over Parker using her. "Pangs" (the Thanksgiving episode) was too preachy for me. "Beer Bad" was dumb but it had one of my favorite bewildered-Xander lines ("Giles, don't make Cave Slayer unhappy.") I didn't particularly care for the whole sci-fi/fantasy crossover but I could deal.