Dice the potatoes and carrots and put them to boil in salted water. While they're doing that, dice the zucchini and onion. Add them as well.
Once the potatoes soften enough, remove them from the stove and mash them. Return to the stove and add the butter and sour cream.
Stir energetically with a whisk until the cream and butter melt. Optionally, you can add a few drops of lemon.
I've never used these particular vegetables for a cream soup before but from now on it's the one I'm going to be making most often for my family. Enjoy!