There are 495 different level configurations for a 4-man party. For example, all level 1, all level 2, three level 3s and a level 6, etc.
Of those 495 combinations, only 2.4% result in a scenario level of 1. The others are:
level 2: 26.1%
level 3: 48.5%
level 4: 21.6%
level 5: 1.4%
There are too many variables to really figure out how often you will actually play at each level though.
Nice mathing! So the potentially most common level is 3. Interesting. Now that's just based on the sheer number of combos. IRL, characters will be retiring at different rates based on their life goals and such, so how often level 3 is used versus reality could be different. It will be fun to play the game over time and see if it averages out that way or not.