I have read J Scotts introduction to estimating ARV and understand you need to look at comps and use the method where you compare similar properties.

Comparable vacation rentals in this area in summer fetch $500+/week. 

Comparable rents are $3,500-$4,000Do I short-term (Airbnb) due to my close proximity to year round destinations like Disneyland, Beaches, SoCal?

@Jaysen Medhurst How do HELOC rates compare to a commercial or conventional rates?

We had a super detailed letter from the dad-in-law on how the husband made a massive game changing contribution to the business, whole 9 yards, didn't cut it.We don't have all the details of your scenario, but when you mention dad as both employer AND cosigner, it starts to share elements -- "the dad obviously really wants you to get the loan or he wouldn't be co-signing, can we really trust those paystubs from said dad?" 

Are there other loans that compare to the FHA loan that would allow me to buy all 4 units even though they are zoned individual town homes?

Start asking different professionals these questions and compare their solutions - stick with the professionals who have the best "niche" advice.

You might get a little more, since they are side-by-side, but how does this look compared to a 4-plex?

We thought the terms were pretty good, but wanted to get the opinion of a more experienced investor.then all I can say is go for it your not going to get better always go with the local community bank even if its a tad more. at least U know they are for real compared to those scammers that are so previlant these days offering you terms that are not real in hopes of stealing money from you for up front due diligence.  

I have seen plenty of posts discussing growth in the area and some basic analysis shows that properties are certainly priced favorably (price to rent ratio, etc.) when compared to my home market.