SoCal pricing is high so maybe the number work better on a primary residence rehab hack on a 4 plex.

I am wondering whether it is better to save up cash  (will take longer to do) to use toward a down payment or use equity in my primary residence for the down payment, then take out a 2nd mortgage to finance the balance on my investment property.

So I helped my sister moving into a apartment by getting on her lease but I don’t live there.If I were to buy myself property back by an fha loan would I run into problems even though I’m attached to her lease but I don’t live there and I’m willing to live in the fha back home as my primary residence Again I dont live at the apt but I’m on that lease for 8 more months.None of the bills are in my name either.

Borrower no longer residing in home as primary? 

I'll admit I don't plan to scale with this method, but my house-hacked primary residence is a condo, and my first 20% down buy and hold rental was a condo.

My only experience purchasing real estate so far is for my primary residence using a bank.

Also, when you move out, you can either rent the doce where you used to live for additional cash flow or sell the property and do not pay capital gains on the portion of the property that you used as your primary residence. 

This structure will let you capture the capital gains on the land thus far without it triggering capital gains taxes (assuming you don't trigger any of the limitations on the sale of your personal residence, or the gains are more than the exclusion). 

However, if paid off, it would significantly dig into my savings, to the point it may or may not effect the upcoming primary residence purchase. 2.

Aside from my personal residence, I don't have any deals myself.