,On a R3 lot with 2 units currently with a lot size of 3750 sq ft, Can I build 2 additional ADU's or 2 units and make a fourplex ?

What is the size of your retail space?

The best way to price your listing effectively is to look up other comparable Airbnb/VRBO listings in our OBX market (size, bedrooms/sleeping capacity, bathrooms, town/neighborhood, proximity to beach/sound, pool/spa, amenities, elevator, etc) and price your property with competitive/similar pricing. 

You may also want to visit Harvard's joint center for housing studies2019 JCHS State of the Nation's HousingJCHS data and charts

From there I’d save double the amount in reserves for the following three months after purchasing and then allocate a certain percentage to reserves every month thereafter (depending on age, size, and other details of the property).

All bedrooms are pretty good size and we also have an office space. 

I am actually signing a contract today for a medium sized multi family unit.  

Any restrictions due to lot size? 

Using a price per SF will have you off on both these examples and they are identical in size.

If the deal is large, then I charge more based on the size of the deal.