All the parties involved would be interviewed privately by Human Resources (HR) including the "boss" and 4 workers directly involved, as well as any workers who heard and/or saw this happen.
The other workers would be asked what they saw and heard, and their own feelings about such comments or have they experienced things that might be racist and what.
Then HR will determine if more info is needed, such as if previous situations have occurred between this boss and other employees. If HR determines that the source is mainly the boss because HR sees a pattern (may have already tried to resolve before with similar situations involving other employees), they may talk to him/her privately to put him/her on notice and review again with him/her what the work rules and how they specifically apply in this situation).
HR would also review with each individual privately what the work rules are and how they apply and also provide training in general for all people in that work area.
Then HR schedules group training by an outside source for all directly involved and the rest of staff. In some cases, scenarios will be re-enacted by outside source staff and then allow the people attending the training to offer suggestions and/or ask questions on how to resolve without referring to any specific event.
Then everyone will be encouraged to do their part in making this process of handling issues a success and that HR will be checking back with individuals to see how it is going.
Regardless of "fault", the yelling, swearing, and inappropriate comments by all will be addressed by HR.
Unfortunately, in the situation regarding President Trump and the 4 congresswomen, no one to date seems to have a way to use someone or some group to mediate this problem. And some that could, are reluctant to step up for various reasons.