The Frog theme is played when Rosella sees a frog sunning itself on a lily pad in the pond. Later in the game, after the frog leaves, the theme is no longer played. The song was composed by William Goldstein and features a blasting horn with a mysterious harmony.
King's Quest IV: The Perils of Rosella (DOS)
Five different types of output were designed for the DOS release, each output corresponds to a different group of devices, the Roland, Yamaha, OPL2, 3 Voice, and PC Speaker.
The optimal MT-32 output uses a brass melody with a mysterious harmony. The OPL2 can't handle the low pitch brass and makes the brass just sound like farting. The 3 Voice as usual loses all instrument timbres but keeps the melody and harmony. And the PC Speaker, with just the melody, loses the feel altogether.
The file is sound.028 in the resource files, it does not loop.
King's Quest IV: The Perils of Rosella (A2GS)
The Apple IIGS port plays the song on the built-in ES5503 audio chip.
King's Quest IV: The Perils of Rosella (AMI)
The Amiga port plays the song on the built-in 8364 audio chip.
King's Quest IV: The Perils of Rosella (AST)
The Atari ST port included at least two types of output, one for Roland devices and one for the system's built-in YM2149 audio chip. Since the Roland output is the same as the DOS release, it will not be recorded.
The Apple II port does contain music, but this song was excluded to conserve space.