The Fourth Amendment (Unreasonable Search & Seizure) is pretty good.
I’ve always been fond of Beryllium, too.
Civilization IV
I’m unclear as to exactly what the OP is looking for – a series where the fourth installment was the best of that series, a series where the fourth installment was good at all, or a fourth installment that is better than other fourth installments?
I remember King’s Quest IV: The Perils of Rosella fondly, but I don’t think it was the best game of the series. Quest for Glory IV: Shadows of Darkness was in many ways the best QfG game, except that it suffered from a number of serious bugs.
A third vote for Super Mario World.
(Alternatively, we could call Super Mario Bros. 3 the “fourth” in the sense that Lost Levels/the Japanese SMB2 lies between SMB1 and the American SMB2. But I prefer to think of things according to the American chronology I grew up with…)
I’ll cast another vote for Super Mario World being the best Mario game (I was actually thinking of starting a SMB3 vs SMW debate one of these days…)
Call of Duty IV modern warfare , and I havent played Left1 to 3 but Left4(dead) is a great game
Medieval II: Total War is the 4th Total War game, and the best - at least for another two weeks.
Rocky IV is stupid, but it’s a damned lot of fun if you’re in the right mood for it. The whole James Brown / Apollo vs. the Russian scene is so over the top, and the training montage in the snow is probably the 80sest training montage ever.
Drago’s lines are pretty memorable too.
Is it the best of the 4? Meh. I will say this — if you asked me to pick one to sit down and watch it would be that one. First 2 may have been better movies, but 4 might be the most entertaining.
I would argue that the third book is the best. At the very least, it’s a close top three.
Heh, I must be in the minority on my book opinions. **The Drawing of the Three **was a brilliant book, and can stand alone in the series, far better than the meandering W&G.
Tabitha King’s fourth book Pearl is a masterpiece.
Nice one.
For movie, I’m picking the 4th Star Wars. Not sure if that means Episode 1 or 4 but, either way, that’s the best movie I can think of. 
I didn’t care for Super Mario 61 through 63, but Super Mario 64 was a real breakthrough.
[Perhaps someday Duke Nukem Forever will make this list…]
It’s actually not my favorite of the series, but I’m going with John Updike’s Rabbit at Rest. I love the Rabbit series and it it was cool that he would toss one out every 10 years and it would roughly follow his own aging process.
It’s of course arguable which of the 4 Blackadder seasons is the best, but Blackadder goes Forth is a serious contender. And it ends right.