Dissolve Knox in 1/2 c. cold water.
Bring milk to a boil & remove from heat. Add sugar, knox, vanilla and sour cream. Beat for a few minutes. Set aside but DO NOT REFRIGERATE!
Prepare Lemon Jello with 1 c. boiling water and 1/2 c. cold water. Pour into a 11x15 pan and refrigerate until firm (about 45 minutes).
Spread 1-1/4 c. of sour cream mixture and refrigerate for 45 minutes or until set. Repeat, alternating Jello layers and sour cream mixture.
Traditionally we have put Lemon on the bottom, then Orange, then Lime and then Strawberry on top.