Yes, the debt-financed percentage recomputes. It's based on average acquisition indebtedness over the year against the average adjusted basis, so as the non-recourse loan amortizes the taxable fraction falls, and the twelve-month lookback on the highest debt balance is what keeps you from paying the loan down in December and claiming a low ratio. That mechanic matters most in the year you sell, where a property sold within twelve months of carrying debt drags gain into debt-financed income even if the loan was retired first. Confirm the current computation with a CPA who files 990-Ts, because the ordering rules are where people get it wrong.
Your numbers make the point on their own. Net $9,000 less roughly $8,700 depreciation leaves about $300, and a third of that is debt-financed, so you're inside the $1,000 deduction and owe nothing. That looks free until you notice what depreciation is doing. It's the entire reason the number is small, and depreciation runs out while the loan may not. It also doesn't help on the sale, where the gain is larger and the debt-financed share of it is fully in play.
On the Solo 401(k): a qualified plan is generally outside the debt-financed income rules for real property acquired with non-recourse debt, subject to conditions, and that exemption is genuinely why operators use it. It isn't a wrapper you can just prefer. The plan needs real self-employment income behind it, as @halyard says, and a traditional IRA rollover into it has to be a permitted rollover of pre-tax money. Rolling a Roth IRA into a Roth 401(k) isn't allowed. If the plan later fails the no-employees test, you're unwinding a plan that owns a four-plex, which is not a paper problem. Prohibited-transaction rules still apply in full either way. Have an ERISA-side professional confirm the plan document actually permits real property and non-recourse debt before you fund anything, since many prototype documents don't.
