- if you already have a job and are happy with it
- if you already have a career and are happy with it
- if you have any debt or other financial obligations
- if you have a family or other social / personal obligations
- your definition of "invest"
...I'll go ahead and give my nontraditional "dream big" answer. Chase your dreams and invest in yourself. Master your desired skill. Turn it into an income source. You've got 4 years to do so.
Just for the sake of argument, let's assume your monthly expenses are $2,000 USD. If you're a guy/gal in the western world with little/no debt and no major familial obligations, this is plenty of money to live very comfortably basically anywhere in the world, other than the top ±100 most expensive cities. Skip Tokyo, London, NYC, SF, Hong Kong, Oslo, Vancouver, etc. Go for Lisbon, Berlin, Budapest, Hiroshima, Vienna, Mexico City, Buenos Aires, Bangkok, instead.
At $2,000 per month, $100,000 gives you just over 4 years in runway. This is conveniently the exact amount of time to achieve "mastery" according to the 10,000 hour model. 50 hours a week X 50 weeks per year X 4 years is 10,000 hours. Personally I find the Mastery model put forth by Robert Greene in Mastery to be a good guide.
No matter what your passion is - painting, photography, writing novels, making movies, being an actor, building a startup - there's a pretty good chance that you can figure out how to make a living from it in 4 years. People that have enough time, focus, and financial resources to dedicate toward mastering their craft for years are extremely rare, especially in this day and age.
Of course, if you're already happy with your career choice, then you can ignore all of this advice.