The idea is, for round numbers sake, if you have one million dollars in total invested funds in your 401K, IRA, etc. accounts the firm would offer you a line of credit up to a percentage of your held assets.  

Don't forget about the tax advantages of the 401K and IRA's on top of the free money from matching, those add to your returns that often get overlooked on the investment sites dashboard.

However, if you are over age 59 /12 or no longer work for the employer that offers the 401k plan, you can transfer the 401k to a Traditional IRA and then invest the IRA in real estate. 

@Matthew OrmeYou certainly can use BRRRR under the umbrella of an IRA and the benefits of such a strategy in terms of maximizing return for cash deployed will apply.  

I've done quite a bit of research on buying real estate through a self directed IRA with rolled over 401K funds.

I have a SD-IRA and I need to deploy.

Can we use money from 401(k)/IRA's in this project?

I have set up a Self-Directed IRA and funded it through Advanta in Atlanta.

We have a new Real Estate Fund and need a reputable custodian that can help our investors convert their IRAs to self directed IRAs so they can invest these assets in our fund.

I have a note that I bought from my SDIRA and now I am finding out that I have to obtain an EIN for my self directed IRA.