A 401k is employer sponsored. There is no requirement that the employer be an LLC or corporation. The employer could be as simple as your sole-proprietorship which pays you the income it makes lending money or buying tax liens. This is true with or without checkbook access to an SD401k. I think this responds to Jon’s comment.
Jeff S and Bryan Hancock: So, if I understand correctly, I could take IRA funds or funds in a previous employer’s 401k, and roll them into a solo 401k. Then I create a company and have the solo401k invest the money into that company. (May have things out of order here.) Then company then makes loans, buys tax liens, flips houses, or whatever. That (hopefully) generates some income. I pay that income back to myself as a salary. The income comes to me, not back into the solo 401k. Did I understand that correctly? That’s passes IRS scrutiny and isn’t self-dealing?