I have seen more people torch their retirement plan using this "strategy" than I care to think about.As @Chris Seveney notes, an IRA can be setup with a specialty self-directed custodian so the IRA can be invested in non-traditional assets like real estate. 

Creative investment strategies include short-term rentals, medium-term rentals, nursing/senior living homes, and rent-by-the-room—all viable investment strategies depending on the market that can create significantly more significant cash flow incentives than if the property were sourced as a traditional rental. 2.

They move much faster than traditional banks and usually have fewer restrictions for qualifying.  

that's my real question... the follow up question is... do you think it's worth $3,150 to go from 6.275 to 5.75 on a 30 year traditional loan? 

Generally speaking, the distributions from a traditional 401(k) account are all subject to taxation versus a ROTH account which has some tax-free distributions. 

Pierre Eade Hey Pierre, There are DSCR 2nd Mortgages available if you are having trouble qualifying for a traditional HELOC.

Traditional rental mortgage.

Traditional rental mortgage.

Acquiring traditional buy-and-hold rentals is still competitive but not as much as flipping from what I'm seeing.

This flexibility is unmatched by traditional banks.