The advantage to a line of credit is flexibility, but the trade off is exposure to a rising prime rate.

Why would I put excess share of my precious capital in a mortgage to save 4% APR, when I could buy another rental and earn way more?

The excess cash flow will be over $10,000 per month.Thanks,

Insurance seems a bit low but your in a different area then my limited exposure.

If I invest in real estate, I may be able to get returns in excess of 10-12%. if I invest in stock market I may be able to average 8%.

What is my exposure if I do go forward? 

When all of those constructions will be completed (depending on which asset class), there will be excess supply in the market, throwing off the balance of supply and demand.

They were OK with them but I didn't think they were that great as far as excess cash. 

I'm sure a whole horde of people on BP could come up with a better list than me, but some that immediately come to mind are:1) Natural disaster2) Significant changes to insurance rates (see #1)3) Changes to state or city laws regarding fees, taxes and rates, code requirements, zoning, rent caps, tenant-favorable laws, etc (you have more exposure here being in primarily one city area as well)4) Changes to state or regional labor market, especially in regard to contractorsNow in terms of rental market shift exposure, I think a geographically-centralized portfolio can still achieve diversification within a large enough city or region through properties in different geographic neighborhoods, Class A/B/C areas, tenant demographics, and general rent amounts (high/medium/low in the market).Now whether those (unlikely) risk factors are worth giving up all the home field advantages you've built up is a lot harder to answer, but that's every investor's decision.

I would look at investing your $150k in something that gives you monthly payments in excess of your loan payment and use that money to pay off your home loan.