The whole cast is still together. However, as you know, Captain Aceveda was getting promoted to council. He is still a character in the show, but he is no longer captain of The Barn. Season Three focused a lot on who would take his place. For most of the series, it seemed set to be Claudette, but her moral crusading damaged her standing in the eyes of the powers-that-be, who decided that Glenn Close's character would be a better choice for Captain.
I personally am a huge fan of "The Shield". It is still my all-time favourite TV show. I can just rewatch episodes over and over and they're still as good. Vic Mackey is just the greatest TV cop ever, though I think Dutch and Shane are great characters as well.
Season Three isn't coming out on DVD over here in the UK until near the end of the year, and I have no idea when Season 4 will be on TV. You Americans are so lucky!