Mostly reposted from the KQ Companion Encyclopedia thread, with some additions for context:
I believe the KQ Omnipedia has already noted this, but the Magic Fruit which Rosella seeks out in order to save Graham's life in KQ4 is taken in part from the Tale of the Three Princes in the 1001 Arabian Nights, wherein a prince, one of three brothers, saves the life of a dying princess with a magic fruit.
(Funny thing: there was a TV miniseries version of the Arabian Nights about a decade ago, which featured this story... but in the TV version, the prince must save his own father's life, instead of saving a princess.)
Looking back at the Narnia books, in The Magician's Nephew there's a magical tree in a walled garden, on which grow silver apples. The tree is guarded by a phoenix nesting in its branches. The book's hero, Digory, gains a fruit from the tree and gives it to his seriously ill mother, who is healed by eating it. CS Lewis probably got the idea from the Arabian Nights story.
There's also a Golden Apple from the Tree of Life in The White Snake, a story in The Green Fairy Book (the same tale, in fact, which provided the magical white snake, the talking-to-animals motif, and even the helpful Ant King seen in KQ5). The hero of The White Snake is told by a princess that he must bring her such an apple if he hopes to win her hand; with the help of animals he befriended earlier, he succeeds in the quest, wins the princess's heart, and becomes king.
The Green Fairy Book is a collection of fairy tales put together by Andrew Lang in 1892, and in fact is one part of a 12-volume set of Lang's color-coded fairy tale books. It's very interesting, in fact, and seems to have provided a LOT of inspiration for KQ.
For instance, the tale of Rosanella features a princess named Rosanella, daughter of Queen Balanice. Hmmmm!
In the book's tale Heart of Ice, the queen of the fairies is named Genesta, and it is she who guides the hero, Mannikin, through many perilous obstacles. Double hmmmm!
Also, in Prince Narcissus and the Princess Potentilla the hero uses a magic ring, which when worn grants invisibility, to court a princess without being observed by an evil enchanter, who also covets her hand. This may well have inspired the magic ring of invisibility in KQ1. (As in KQ1, and most unlike the One Ring in Lord of the Rings, this ring is not malevolent in any way.)
There's also Prince Featherhead and the Princess Celandine, whose heroine hails from the Summer Islands. Possibly this was an inspiration for the Green Isles, home of Princess Cassima, first mentioned in KQ5.
(Cassima's name is probably taken from the story of Ali Baba and the 40 Thieves, from the Arabian Nights--Ali Baba's brother is named Cassim.)