The two biggest things that drive the US economy are housing sales and car sales.  

Do you have enough money left over from your home sale to finance the 4plex as an investment property (additional 10-15% downpayment.) and then still have enough left over for a downpayment on a new home?

If you are a veteran you can get 10% discount off their sales price too and often free delivery.

Her father is on board with the sale and even generously offered to give us a portion of whatever the net sale is.

There are a lot of great sales pitches out there that will offer you a "once in a lifetime opportunity" - lets be real, if this was such a great opportunity why would they need your money...

All your customers probably know you, know your product, and know the experience. 

Back to finance question, would it be a better decision to take the funds from the sale of the 2 acre lot and pay off the current mortgage (~$150k) and live in the current home for a year or so (in hopes that interest rates will go down) then refinance that take out $400k from equity (~$750k) to build new house? 

What do you plan to do with the proceeds from the sale if you were to sell? 

Also, you can take a look at what's on the market now in the surrounding area to see if there are any other comparable mobile homes for sale.

You said you provide a great product, so get paid what the market is willing and able to pay.