A Main Character is:
- The character through which the audience experiences the story, and...
- The character is dealing with "her" personal issues independently of everything going on in the Overall Story throughline (the efforts to achieve the Story Goal).
Though it is "possible" that the group is the MC, it is unlikely that is the case. From what little you've described of your story, it doesn't appear that you have an MC in its current form and could benefit from making one of your story's players also the MC.
The overall story characters (such as Protagonist, etc.) are part of the Overall Story throughline and work toward (or against) the achievement of the goal. If you want to tell your story effectively using only these four players, I advise you to break out of the character "archetypes" and create complex characters. Ideally, all of the motivations found in the Build Characters window should be represented by one character or another. Since you only have four characters, you should divvy the sixteen character elements between these characters. You may do so evenly (4 elements each) or unevenly. I highly recommend AGAINST putting two character elements that appear in a dynamic pair (diagonal) relationship into one character. For example: Do NOT put both logic AND feeling into one character.
If you do the above, you will have some OS characters FOR the story goal, and others AGAINST it. Without this tug-of-war of "for" and "against" you wouldn't have any conflict in your story--it would merely be a narrative of events without much to move it along.