Place each flavor of Jell-O gelatin into separate bowls. Add 1 cup boiling water to each bowl and gently stir until the gelatin has dissolved.
Transfer each Jell-O mixture to an 8x4-inch loaf pan. Chill in the refrigerator until firm, about 3 hours to overnight.
Use a knife to cut each Jell-O flavor into ½-inch cubes. Add cubes to a 12-cup Bundt pan, being careful not to break them.
In a large bowl, dissolve the unflavored gelatin in the remaining 4 cups of boiling water. Let cool slightly, then whisk in the sweetened condensed milk until well combined.
Let sit until cool to the touch (to prevent melting the Jell-O cubes), then pour evenly over the Jell-O cubes. Place in the refrigerator overnight until fully set.
Carefully unmold and cut into slices to serve.
