+++Warning, this spoils the major cliffhanger from the end of season three, but does not give away any major season four spoilers+++
Season four of Charmed picks up a week after the events that ended season three, with Prue and Piper being laid out by Shanx's blast. We learn that Prue died from the blast, which was, of course, a way to write Shannen Doherty out of the show. To preserve the whole concept of the Power of Three, a new, half-sister, Paige, played by Scream's Rose McGowan was revealed. Paige was the child of Prue, Phoebe, and Piper's mother Patty and her Whitelighter, Sam, who was given up for adoption as a baby. The first handful of episodes were focused on establishing Paige as a character and Paige learning about her sister's existence and the fact that she is a witch. Then, it pretty much followed the same procedural-serial storyline combination that the first few seasons had. I would say this season's storylines were more serial than they were monster-of-the-week, with major storylines about Phoebe and Cole's relationship, and Piper's emotional breakdown after losing Prue getting multi-episode arcs this season. The cast turnover did not end with Shannen Doherty leaving as Dorian Gregory was basically used as a recurring character this season (although he was still shown in the opening credits of the episodes he was in), but his character would go 3-4 episodes at a time without making an appearance.
For those who get the Blu-Ray set, it is basically a MOD set. You just get the episodes and that is it. You do have the option of a German or English language track and German or English subtitles, but there are no extras, which honestly, given that the season aired in 2001 and the blu-rays were not released until years later is not surprising. This season does have a lot of good guest stars including Daniel Dae Kim (who would later star on Lost and the Hawaii Five-0 reboot), Robert Englund (the original Freddy Kruger), Ray Wise (from Twin Peaks), Coolio, Dave Navarro, Armin Shimerman (from Buffy), and Bruce Cambell.
Overall, the beginning of the season was a bit uneven as the show transitioned from Shannen Doherty to Rose McGowen. As many are likely aware, Doherty and Milano did not get along (how much neither of them has ever said, although they both acknowledged tension as has Holly Marie Combs) and there has been speculation for years whether Doherty was fired or quit. Speculation is that it went far beyond tension and they actually hated each other, and Milano gave an ultimatum to the showrunners that either Doherty goes or she would. Milano has always denied that ever happened, but given that Doherty directed the season three finale and was gone by season four, it seems that things came to a head when Doherty was in charge. It does seem like the show did her and the character of Prue dirty by not having her appear to give her a proper sendoff. At any rate, once the character of Paige was figured out by the writers and McGowan got her footing with the character, don't think the show lost much steam. It seemed like Milano, McGowan, and Combs worked well together, although if you have seen Milano and McGowan's social media interactions, it seems that there is no love lost between those two either.
There are things about the show that would probably not be done the same now as they were back then. There was definitely still some mooning over men that would probably not be written the same way, and they were definitely putting McGowan in a ton of skimpy outfits as they had put Milano in during seasons 1-3. Of course, this was back when McGowan was hot and not identifiably crazy, so she was put in crop tops and midriff-baring outfits constantly. And, the special effects were still not great by any means, and sometimes the HD transfer makes that even more noticeable. That said, if you enjoyed the show's first few seasons (unless you only watched it because Shannen Doherty was in it), you will probably enjoy this season as well.