Time to get serious - How should I use cash and equity to invest?

[Ben Rodriguez]

I currently own 4 properties - three SFHs and a duplex which I recently closed on and am currently moving into.  Up until recently, I had purchased each of my properties with the intention of living in them for a couple+ years, then renting them out when I found another home. However, after being on bigger pockets, reading books and listening to podcasts, I want to start investing in a more serious fashion.  So my question is how best to use money and equity to that end.

A little synopsis of my current standing, for context. The rent from the other half of the duplex just about covers my mortgage on it. The SFRs do not cash flow but two of them have built up a bit of equity. The one I'm moving out of is conservatively valued at $152k and I have $64k remaining on the loan. I plan to rent it out and it should fetch $1050 if not $1100. Also, I recently opened a $55k HELOC on it but have not used it. The property other, my very first home, is conservatively valued at $185k with $75k remaining on the loan and is currently rented at $1100. Market rent for this one is between $1200 and $1300 but I have excellent tenants and don't want to raise rents just yet.

Because I work a well paying job and I'm somewhat frugal, I wasn't concerned about having cash flow so much as having the properties pay themselves off by retirement, which is about 7 years away.  I had therefore put these on a 15 year mortgage, which meant lower interest (one at 2.875% and the other at 3.57%) but higher monthly payment.  After mortgage, insurance, and taxes, I'm left with only about $120 a month. 

As far as the 3rd SFR, I don't know whether to count it as an investment, since I bought it at the request of my parents. I put this one on a 30 year loan so the payments would be lower for them. They give me just enough to cover the mortgage, with a bit left over, which I use to pay down the principle. However, it has appreciated about $30k since I bought it in 2017.

All four properties are newer (2004, 2006, 2013, and 2017), so there haven't been major expenses, though I'm sure some are due.  

So, to summarize, I have available a $55k HELOC and, taking into account the 75% LTV rule, a combined $90k in usable equity in the other two properties. I also currently sit on about $50k in cash, and I have about $23k un-leveraged debt and a $17k loan from my IRA (yes, I know, I know) which I had used to buy my previous home.

My question then, how can I best make use of this?  I assume most will say to use the liquid cash to pay off the debt, but I'm also particularly interested in hearing what I should do with the homes with equity.  Would it be prudent to: 

A) refinance for cash flow and use it to...?  

B) cash out refi to reinvest in cash flowing properties (I'm interested in quads)?  

C) pay them off sooner by paying of my debts, using the remainder to pay down the principle, while also contributing what would have gone towards monthly debt payments?

Or, D) - Z) ?

Thanks in advance and sorry for the long-windedness!