A solo 401k retirement investment account is the best plan for business owners and self-employed individuals. The most distinguishing feature of the plan, when compared to other traditional retirement plans, is the contribution. In addition to the employer contribution, this plan allows the employees to make contributions. It is also known as individual 401k, solo-k, uni-k and One-participant-k. This plan enables the contributor to use their retirement savings to invest in various investment opportunities such as real estate, tax liens, tax deeds.