@Kevin Parnella If you are changing jobs, you will have the option to rollover your current 401k either to a new employer 401k or an IRA in your own name.If you roll to a 401k, you can borrow from the plan as you note.

Refinance into a new 15 or 30 year, roll over balance, cash that comes out is 90K leaving 40K in (80% financed).- Next day, boom, crash, property value drops to 50K (50% of what you bought it for); you owe 160K to the lender however.This is obviously a less than great situation to be in and you're definitely minus equity (as opposed to the 20% that the lender wants) - but, what is the lender going to do - demand you pay up and come back to 20% based on the new assessed value and foreclose on you if you do not?

you CAN use your 401k to invest in RE" (rollover SEP-IRA, borrow, cash out, etc.) but nowhere can I find out if you SHOULD use it.

I find most tenants are quite docile, especially the long stay ones and you can get them to roll over quite easily if you are nice to them.

You may not be actively involved personally and provide services to your IRA, as that would essentially amount to making un-documented contributions to the IRA and artificially boosting the sheltering value.An alternate and entirely different structure known as a Rollover as Business Startup (ROBS plan) exists that allows for retirement funds to be utilized to capitalize an active business in which you are hands on engage. 

@Gary OnstadFollowing 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 (IR 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.)

Yes, these contributions must be sourced from self-employment earnings.There is no limit on how much you can actually invest once the account is funded with rollovers, contributions, or a combination of both.

Could you roll over wholesale profits to a 1031 so you don't have to pay capital gains?

They can roll over funds into a self directed IRA and either invest with you as a partner or Private Lender.

It will at be least another day before the account is finalized and at least 2 more before the rollover happens.