Continuity mistake: Rumplestiltskin looks completely different in this movie than he did in the previous film Shrek The Third; Prince Charming momentarily talks to Rumplestiltskin in the tavern during the third film, and the character looks completely different, has a different voice and different demeanor.
Plot hole: In the second Shrek movie, we find out that Fairy Godmother and the King had a deal that Prince Charming would save Princess Fiona from the keep, hence becoming her true love and breaking the curse. In the fourth movie, we start with Rumple monologuing about how he almost had the Kingdom by signing a deal with the King. Why would the King be so desperate to try and reverse the curse by signing a deal when he knows he has a prior attempt through Prince Charming, assuming it would take the same time for the messenger to return with news about Charming and Fiona either way?
Plot hole: Rumpelstiltskin is at a low point in his life, so he's in the back alley eating from the garbage cans. He gets the idea to trick Shrek so he can take over Far Far Away, and bribes him with a rat cooking in the oven and eyeball martinis. Why is Rumpelstiltskin eating out of the garbage when he has a plump rat with vegetables roasting and eyeball martinis? Even if just the vegetables appealed that's better than garbage.
Continuity mistake: In the scene where Shrek goes back to the swamp after signing the contract, he realizes that Fiona isn't there and pulls out the contract to read it. He then notices something flying overhead and looks up to see the witches. In the next shot, we see no contract in Shrek's open hands. (00:22:30)
Continuity mistake: Fifi the goose changes size throughout the film. From small enough to fit on Rumpelstiltskin's lap, to big enough to pull the cart.
Continuity mistake: In the scene when Pinocchio arrives to request Rumpelstiltskin to convert him into a real kid there's nothing on the table but goblets and kind of cakes, after Rumpelstiltskin asks to the wolf his wig, he suddenly takes the contract from the table and on it appears the feather and ink that Pinocchio uses to sign the contract.
Continuity mistake: It was morning when Shrek enters the castle, because Donkey shouted to Shrek, "and find us some breakfast." And after leaving the castle, Donkey found a waffle and got pulled toward the ogres' hiding place. Once inside the hiding place, Shrek tried to take Donkey away from becoming a "dinner" by saying "Your dinner is my friend." Immediately Fiona showed up as an ogre, which confirms that it has suddenly become night. (00:36:30 - 00:41:05)