He offered to sell it to me with owner financing for $400,000 and only required 10% down, so I jumped on it.

@Sung Yu You can 1031 exchange your fourplex into a single-family home, provided it’s initially used as an investment property (rented for 1–2 years) to meet IRS requirements.

Did you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?

If you're concerned about whether the rates match what was rented, we recommend asking your property management company if they can provide access to the booking details.

And I believe they require 30-40% down. 

Did you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?

@Brady Bitter these funds have margin requirements. 

This is prime real estate, and is just about the last piece of land in this area.I have begun to develop a commercial property that I own with contractor garages, which is the extent of my experience in this space.I'm trying to get a sense of how to structure something like this, with using LP's, and also partnering with a builder, and carving something out in that regard.Does anyone have recommendations on who is currently working in this space and could provide some insight?

Can anyone recommend a HELOC lender for a SFH investment property in Los Angeles, CA?

To avoid reducing your lifetime exemption, you could gift equity incrementally, but this requires proper legal structuring (e.g., fractional ownership transfers).