Dissolve 2 envelopes of Knox gelatin with 2 cups of boiling water in a bowl. Mix with sweetened condensed milk. Mix well, and set aside. Will be approximately 3 cups.
Divide the 4 boxes of Jello into 4 separate bowls. Add 1/2 envelope of Knox gelatin to every bowl. Add 1 cup of boiling water to every bowl and stir the mixtures until well dissolved.
Spray a 13×9 pan lightly with oil. Carefully pour 1 bowl of jello mixture into pan. Place pan in freezer, making sure it's level, 8-10 minutes for the first layer, just until there is a light fingerprint when you touch the top.
Pour 1 cup condensed milk mixture on top, and freeze for 7 minutes. Continue layering and freezing for 7 minutes for each layer. Don’t freeze too long between layers or they will not stick together as well.
If needed, microwave bowls of jello in 10-20 second increments, to keep from coagulation while you wait for each layer.
Once layers are done, cover and chill in fridge until fully set. Slice into squares with a sharp knife and serve on a platter, if desired.
Notes
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