-New construction costs of $200/sf @ 1,500 SF total, (750 per door) = $300k. 

Each unit has one for trash and one for recycling so there are six in total.

-How many offers were there total?

This one is obvious, but the farther away, the less connected you will be to the actual property.

In regards to the tax assessment, it might make more sense if you prorate the land value over the total value and apply it to your purchase price 

Your cost to buy a property, your TOTAL cost, is only the cash you put into it.  

Well now year 1 has an extra deduction of $800 ($2,000 total) causing tax to go down that year.

The tithe was given on "the increase" not the total, and I think that is a reasonable look at how to determine what to give on....but again it is a matter of the heart, it is not a law to be followed. 

You'll most likely never recover your cost (cash).The cost of a 4plex should be less than 4 SF's, but that also means your total property value will be far less for the amount of cash (cost) you put into it. 

Could pay cash for everything to get built and lease up to break even and then cash out refi to pay them back and possibly (obviously could be worse too) at a lower interest rate then now.