Snap Freeze is an upgrade for the Ice Tower that was introduced in Bloons Tower Defense 3 and has appeared in every sequential BTD game since. When a bloon is hit by the Ice Tower, it will pop a layer of that bloon and then freeze the layer below in the process. In BTD4, an Ice Tower with Snap Freeze appears identical to the regular Ice Tower, albeit with 3 additional Upgrade Marks by the side.
In Bloons TD 4 it is the 3rd upgrade for the Ice Tower, costing $340 on Easy, $400 on Medium and $430 on Hard. This upgrade had been improved to pop all bloons in the Ice Tower's radius (other than White and Zebra Bloons), rather than popping only some bloons.