Illinois does not have a Good Samaritan law which requires one to stop a fight they are watching. However, the person not doing anything could be charged under a principal theory, whereby he aided, abetted, or encouraged the actions of the people who participated in the fight. This would be very factually driven and depend on tiny nuisances in the circumstances that led up to the fight. The fact that this was planned out makes that argument stronger. A good attorney could fight this if they ever charged him/her.
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He could be arrested for planning and actually showing up. He could be charged with the same offenses as the ones who actually inflicted the blows. Note, I said charged, and not convicted. As the previous attorney responded, this is a very factually driven case and the interested party should contact an experienced criminal defense attorney and discuss this case with him/her and refrain from providing further details on the internet, including this site.
This response is being provided for information purposes only and does not constitute an attorney client relationship. Furthermore, I am only licensed to practice law in the State of Illinois. While there are oftentimes similarities between States' laws, there can also be large differences. You should not rely on this response as legal advice and are highly encouraged to speak to an attorney licensed in your State for an accurate legal answer.
I doubt that there would be a criminal charge for not intervening but there might very well be criminal charges. Solicitation and conspiracy charges are possible, and #4 could also be thought deeply enough involved in the planning to warrant charge and possible conviction for whatever offenses ##s 1-3 could face. #4 is, in other words, in a very serious situation and would do well to retain an attorney and to avoid talking to the police about this matter without legal representation . . . and should not discuss it with anybody else or post about it on the internet.