I would also be interested in learning more about after-tax 401k contributions. My wife and I both have IRAs from previous rolled over 401/403s, which would need to be dealt with before making backdoor Roth contributions as well, and while my 401k and her TSP both accept transfers from tIRAs, I have a bit of a bias towards keeping those moneys in a Fidelity account. I have also been reading about solo/individual 401ks, and while neither of us has a personal business or contractor type work, I was wondering whether that type of account might be a good way to recharacterize the tIRAs, allow the flexibility to invest of investing outside our current 401k/TSP available funds, and get the benefit of having something in place for additional saving if we were to really seek out income outside of our jobs. Looking at the forms for starting a solo 401k at Fidelity (Vanguard does not accept rollovers or transfer into their accounts I believe), it looks like the only requirement that you have to make to show you have a business is give them your EIN, which you can get online through the IRS. I'm just wondering, since I couldn't find a detailed answer after a cursory google search, what the minimum requirements are to actually claim a sole proprietor business? Does selling a few things on Amazon count, and including that info with your EIN on your tax return? Maybe it's more hassle than it's worth if you don't have an actual "business", but I haven't seen anyone discuss using it for this type of purpose, so wanted to throw it out there for the OP and for anyone else to comment and maybe disabuse me. Thanks!