Perhaps some cars on blocks or couches on the front porch.

Most of them are sold individually so buying the entire 4plex doesn't come up as often.

You can move in and get renters to live with you just keep in mind most cities only allow 3 unrelated individuals living at one address.

You still have several options, depending on if you want to invest directly in individual properties or be a bit more removed:Full-service turnkey investment: Should be mostly passive after you do your due diligence and pick a provider, but don't skimp on your homeworkYou choose which props to purchase, but have no control over tenanting choices, some say in large maintenance expensesAvg cost for solid B/B+ prop in Birmingham (and some other markets but this is the one I have data for, since it's my market) is about $100k per door; you'll pay market price for a tenant-ready, fully rehabbed propertyPartnering with someone who does the on-the-ground stuff while you provide capitalCan be passive if your partner really knows their stuff, but more likely you'd be fairly involved with the choices madeMore control since you call the shots with your partnerYou can pick which markets and price points you're interested inPotential for higher returns (ie buying distressed and then forcing equity through renovation) if your partner is experienced and can execute consistentlyInvesting in a syndicateMany investors pool funds to invest in much bigger projects like commercial space or large MFRs, or in larger portfoliosVery passive, investors are not responsible for project vetting or management, but you have no controlMay have higher bar for entry, some syndicates require large investments and you'll need to have liquid cash on handBuying shares in a REITLike an ETF but comprised of real estate investmentsVery passive, but no control over which assets are held in the fundHighly liquid, easy to buy and trade, lower bar for entryEverything is a trade-off between passivity and control, time and money.

You'll need to separate your assets (either by using a Series-LLC or individual LLC with assessed distribution of assets by LLC) and you'll need to separate your passive (asset holding entities) from active operations (the property management side).After you took care of all this you can look into other more complicated/expensive strategies like you describe (equity stripping).Here is a diagram to help you on this quest - talk with @Scott Smith for professional advice on this:

One could argue that individuals could use a similar strategy to "vet" areas they are less (or not) familiar with.

(Just like apts.)If you have any park owned homes you will have to value them individually.

If so, is it through each individual city or a county? 

Get a strong operating agreement drafted that outlines the purpose of the business and your individual roles in detail.Take it one at a time and grow from there!

or the individual's income?