Think of yourself as a fund manager putting that capital to its best use.There is a program known as a ROBS (Rollover as Business Startup) that can be used to fund a business, but the business must be providing a product or service (like a restaurant or real estate development company).  

Not sure the product type (office/retail/industrial/MF) or tenant profile (long term leased, near term rollover, vacant, etc), but you should dial in the following assumptions for your proforma:Market rentsLeasing costs (TI/LC/free rent)Leasing downtimeInitial capital costs (structural/repositioning - if any)Operating expensesThen run a cash flow analysis and figure out your hold period/exit cap/etc.

After year one, taking into account you have 50K at the end of the year plus the 6K you made on the rental property you should have 56K in the bankYear 2: The end of year 2 you should get another 50K + 6K from the rental + 56K roll over for a total in the bank of 112K. 

While this may be true, the salary deferral you would be eligible to put into that Solo(k) would be tied to the income you earn in that LLC - you can't take earnings from another fulltime job and claim them as salary deferrals into your Solo(k).Because of this, most people fund the bulk of their Solo(k)s with rollovers from other existing plans. 

We own two properties outright using SDIRA funds and have built up about 40K in the 'checkbook' from rental income, and are going to use a portion of that, along with a non-recourse loan, and also roll over some more funds from our traditional IRAs into the SDIRAs.

Unfortunately this happens with major brokerage firms and banks such as Vanguard, Fidelity, Wells Fargo, etc. but once that rollover is completed you will be in the "driver's seat" of your 401k (as one of my clients said) and can fully appreciate the power of "self-directing".

See the following IRS link.https://www.irs.gov/Retirement-Plans/Employee-Plans-Compliance-Unit-(EPCU)-Completed-Projects-Project-with-Summary-Reports-%E2%80%93-Rollovers-as-Business-Start-Ups-(ROBS)

Following are the similarities and differences between the solo 401k and the self-directed IRA.The Self-Directed IRA and Solo 401k Similarities Both were created by congress for individuals to save for retirement;Both may be invested in alternative investments such as real estate, precious metals tax liens, promissory notes, private company shares, and stocks and mutual funds, to name a few;Both allow for Roth contributions;Both are subject to prohibited transaction rules;Both are subject to federal taxes at time of distribution;Both allow for checkbook control for placing alternative investments;Both may be invested in annuities;Both are protected from creditors;Both allow for nondeductible contributions;Both are prohibited from investing in assets listed under I.R.C. 408(m); andThe Self-Directed IRA and Solo 401k DifferencesIn order to open a solo 401k, self-employment, whether on a part-time or full-time basis, is required;To open a self-directed IRA, self-employment income is not required;In order to gain IRA checkbook control over the self-directed IRA funds, a limited liability company (IRA LLC) must be utilized;The solo 401k allows for checkbook control from the onset;The solo 401k allows for personal loan known as a solo 401k loan;It is prohibited to borrow from your IRA;The Solo 401k may be invested in life insurance;The self-directed IRA may not be invested in life insurance;The solo 401k allow for high contribution amounts (for 2015; the solo 401k contribution limit is $53,000, whereas the self-directed IRA contribution limit is $5,500);The solo 401k business owner can serve as trustee of the solo 401k;The self-directed IRA participant/owner may not serve as trustee or custodian of her IRA; instead, a trust company or bank institution is required;When distributions commence from the solo 401k a mandatory 20% of federal taxes must be withheld from each distribution and submitted electronically to the IRS by the 15th of the month following the date of each distribution;Rollovers and/or transfers from IRAs or qualified plans (e.g., former employer 401k) to a solo 401k are not reported on Form 5498, but rather on Form 5500-EZ, but only if the air market value of the solo 401k exceeds $250K as of the end of the plan year (generally 12/31);When funds are rolled over or transferred from an IRA or 401k to a self-directed IRA, the amount deposited into the self-directed IRA is reported on Form 5498 by the receiving self-directed IRA custodian by May of the year following the rollover/transfer.Rollovers (provided the 60 day rollover window is satisfied) from an IRA to a Solo 401k or self-directed IRA are reported on lines 15a and 15b of Form 1040;Pre-tax IRA contributions on reported on line 32 of Form 1040;Pre-tax solo 401k contributions are reported on line 28 of Form 1040;Roth solo 401k funds are subject to RMDs;A Roth 401k may be transferred to a Roth IRA (Note that from a planning perspective, it may be advantageous to transfer Roth Solo 401k funds to a Roth IRA before turning age 70 ½ in order to escape the Roth RMD requirement applicable to Roth 401k contributions including Roth Solo 401k contributions and earnings.)

I have been taking depreciation on all my homes - I just deliberately held off beginning depreciation on the fourplex alone because I had already negated my tax liability to zero and thought I could begin depreciating it in the next year, unaware that those expenses could roll over between tax years.

It becomes even more predictable when they're able to begin with a significant 'rollover' stash of cash from another qualified plan.