There are numerous ways to do this. None of them "easy". I recommend the simplest one.
Leave the existing asset alone. Absolutely nothing what-so-ever changes for the existing asset.
Enter another asset that is the 4th unit only. That 4th unit will be 100% business use with an in service date of one day after you moved out, or the date you decided to start renting it out - whichever date is last.
You'll continue with the existing asset claiming 75 percent of the expenses against the existing asset. The remaining 25% will be claimed against the new asset.
The nice thing about doing it this way, is that you don't have to treat this as two separate rental properties. But you *do* have to pay attention to detail so you don't mess things up; especially with the depreciation and allocation of property taxes and property insurance.
Would you like me to cover "the other way" to do this? It's more work on your part - but only for the 2022 tax year when you converted the 4th unit to a rental.
