He took a 401(k) loan to fund the down payment on a four-unit building in a rapidly appreciating neighborhood.

I don't plan on sticking to one method of investing however I do realize that some investors start with Single family and grow into multi-family.

After they asses what all needs replacing or what they feel needs replacing they'll submit to you what they're willing to cover and deductibles and payments etc. 

Publication 15, Employer's Tax Guide, says only this much:...Payments for the services of a child under age 18 who works for their parent in a trade or business aren't subject to social security andMedicare taxes...No definition, and IRS publications are not really law anyway.

There's more to execution than merely coming up with the down payment money.

It 12 years, assuming you make no extra payments, you'll have a paid off condo that cash flows a lot better. 

Last year, on my 9 properties I had negative cash flow after accounting for mortgage payments, etc.

Get listed on all the OTA’s and a direct booking site, use good management software like HostAway, use dynamic pricing software like pricelabs, use WiFi enabled smart locks like Schlage encode, use an email capture system for guests to login to WiFi like stayfi, use those captured emails to do marketing for your direct booking site, and automate scheduling and payments for cleaners and maintenance.

Poor rental payment history is a common cause for non renewal. 

I started by saving up for a down payment which was 5% for the first "second home" property in 2004, then 10% on the next few.