While there are many benefits to using your 401K to fund your business, there are also many cons that can cause you to consider other funding options. The greatest of all the bad news concerns your business itself. If for some reason your business is not profitable (half of all businesses fail in the first year), you will still have to pay back your 401K in order to avoid paying some serious tax penalties. In order to avoid this, make sure that you have a profitable idea.
Second, if you borrow against your 401K, you will slow the growth of your wealth - and potentially hinder your ability to retire on time. Third, when you pay back your 401K, the payments you make are not sheltered from taxes like your initial deposits were. Finally, it can create a habit where you dip into your retirement account every time you need extra money.