Ski-Bird:
Level four is worse than level three.
AnyDice is a neat website that can help you visualize what you're trying to do.
How do you figure? Say you do a level 4 roll, 3d6 and you get some value. Now, convert it to a level 3 roll by rolling 1d6 more. If it is lower than the highest of the original 3 dice, you drop that higher one and take the lower roll. That is, adding that extra die can only reduce the value.
Consider taking it to the extreme: If level X were "Roll 1000 d6 and take the lowest three of the rolls." That will nearly always have a value of 3.
(Maybe the OP has edited since getting your feedback?)
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To OP, your jump from level 1 to 2 is kind of steep. Perhaps Level 1 is "Roll 2d6 and keep the better one."
This message was last edited by the user at 00:33, Sun 12 Apr 2020.