There are 401k plans that allow you to invest into non-traditional investments such as real estate or buying a business. It is beyond the scope of this chapter, and frankly it can be a very bad idea although it sounds hip at your next cocktail party.
A self-directed IRA is easier to setup and maintain, while a self-directed 401k plan is much more challenging. While these two self-directed vehicles, they share similar problems and gotchyas. Here is a recent post on the pitfalls of self-directed IRAs and 401k plans-
If you want more information, we have worked with Equity Trust and New Direction IRA who can create and help maintain these accounts and plans.
Taxpayer's Comprehensive Guide to LLCs and S Corps : 2019 Edition