3-4 hours skills assessment. Consisted of timed questions with math and logic (2 parts). 3rd section is where you learn M (MUMPS), I didn't think this section was that bad and you should do well if you have taken a couple programming classes. Hardest part was definitely the leet code section.
If you did well they'll meet with you for a ~3 hour final superday interview.
First 2 rounds were informational, just prepare to have some questions.
3rd round was a case study where they give you a bunch of tasks for the day and you must tell them how to prioritize them. They will then ask "what if" questions to try to throw you off. My advice would be to have a framework for prioritization and stick with it as new things come up. 4th round is with HR and very conversational. Just be prepared to answer "Why Epic" and why did you pick your major/school.