A type 4 phaser emitter was a small ship-mounted phaser emitter of a type that could be used on shuttlecraft.
Though phasers were not normally installed on type 6 shuttlecraft, they could be fitted for special purposes. The emitters were placed on the forward tips of the two warp nacelles.
In 2368, the Magellan, a shuttle from the USS Enterprise-D, was equipped with type 4 phaser emitters in order to chart the null space in the J'naii star system with a series of photon pulses. (TNG: "The Outcast")
In the original pencil drawings of the hand-held compression phaser rifle, it was also designated as a type IV phaser. 
StarTrek.com uses a picture of the type 8 shuttlecraft
firing its phasers to depict the type 4 phaser, suggesting its armaments also share this designation.