Utilities, taxes (property, local vacation, & income), landscaping, furnishing / decorations / amenities, pest control, & supplies (soap, TP, etc) are the big ones on my STR calculator that you didn't mention.I'd also recommend using the enemy method to determine expected income.

Our locals will negotiate on sight distance but there has to be a reason.  

I recommend looking at B neighborhoods up in your local metro.

Partner with a local lender.

Attend real estate events or join local real estate groups.

Ask local property management companies what they are seeing for utilities for similar fourplexes.I don't know the age of this building but mine are 80s built.

I would recommend attending a local meetup or 2 to get a good understanding what other folks in the area are doing. 

That 4-plex already has a local realtor.

Are there specific networking events, online platforms, or strategies that have proven effective for building relationships with investors?

Do you have a floor & Decor location there locally ?