I also doubt very seriously that any brokerage/custodian is going to allow this kind of loan with 401k assets; it's likely going to need to be a 401k general purpose loan, which is highly regulated (as someone else mentioned). 

Airbnb properties can be hit or miss depending on location, seasonality, and regulations.

Then there’s city regulations—some places heavily restrict short-term rentals, so you’ve got to check the rules.If you find the right spot and automate stuff (guest communication, cleaning, etc.), it can be profitable.

Proper underwriting - this includes an appraisal, loan app and full underwriting to comply with CFPB regulations.2.

If you're going to do it alone, make sure you your due diligence: estoppels, inspection, review, consult with a local attorney familiar with the local regulations, etc. and good luck.

I find it easier this way once you know your regulations and ROI in certain areas.  

And the locals in Highlands have been fiercely debating STR's and regulations for the last few years, so I wouldn't be comfortable buying there.

However, there is no specific holding period contained in the tax code or regulations and there is no "safe harbor." 

As more people placed more properties in STR inventory, and as more municipalities taxed and regulated the industry, the profitability of the overall industry has reverted to a much more aligned (lower) risk adjusted return.  

But I am in FL, and Im sure that scares others, but I am not just talking about natural disasters - just general over-regulation.