@Gary39 , you are correct that MaryK4,'s answer is incorrect.
The term "cost basis" is generally not used when referring to 401(k) plans. The term "after-tax" basis is used instead.
As long as your regular contributions do not already result in reaching the maximum annual additions limit, you can make after-tax contributions to your Solo 401(k) if the plan agreement permits such contributions. The after-tax contributions and attributable gain or loss should be tracked in a separate sub-account within your traditional 401(k).
