Advice Needed, Lots of Equity but unsure of next step

[THANG BUI]

Hello everyone,

My mother and I are considering our next move but do not know the best way.

Properties owned:

4-plex: Value $1.15 million. $530,000 owed at 4.25%, $4000 monthly payments over 15 years. Title: Mother and myself

4-plex 2: Value $1.15 million. Owned free and clear. Title: Only my mother is on the title.

SFR Primary Residence: Value: $900k. $605,000 owed at 4.125%, $3300 monthly payments over 30 years. Title: Mother and myself

HELOC:

Can pull out $83,000 on primary residence - currently at zero balance (prime + 1%)

The problem:

I see a lot of untapped potential in 4-plex 2, which we own free and clear. We attempted to get a line of credit with Wells Fargo but was denied because my mother's income is low ($50,000/year). 

I'm personally high income earner ($100k), however, I am not on the title of the 4-plex 2. 

The question:
What is the best way for us to utilize that equity? Should I add myself to the title then get a line of credit? Should we cash out? If we cash out at 60% value, we can get $805,000. However, that amount of cash scares me into inaction. I worry about blowing all of my mother's retirement assets, even though my mother would like to invest this equity.

What would you do in my situation?

Thank you for all of your sage advice!