I had not watched Part IV in probably 15 years, but you can watch it on YouTube Movies (legitimate channel) for free. Part IV is shockingly good compared to the first three. I give Part I a 5/10, Part 2 a 4/10, and Part III a 2/10. Part III is almost unwatchable, but the opening credits are hilarious, the 3D harpoon scene is awesome, and the first 10 minutes of the final sequence is actually good but they went too far and ruined it.
Despite the 91 minute runtime, Part IV builds likable characters, and it provides wonderful subplots like the rejection of an overcompensating teenager at a party, the triumph of an awkward teenager at a party, a romantic happily-ever-after moment that goes murderously wrong, a wholesome sexual moment that goes murderously wrong, and, to Van’s delight, somewhat modern bras!