I have a 4 plex (two duplexes on one lot) în unincorporated Contra Costa County that cashflows well.  

Similarly, if you put all Class D tenants in a Class A 4-plex, what do you think will happen?

And I know that the tourism infrastructure (restaurants , activities) has some catching up to do aside from the new casino that is opening up.

No thats incorrect, the 4-plex will have a bit of a higher rate but it wont be much.Anything 1-4 Units is pretty similar in pricing.We look at DSCR on both property types.

Should I put heloc on the 3-plex due to the current low rate and use the cash to BRRR?

Similarly, if you put all Class D tenants in a Class A 4-plex, what do you think will happen?

Similarly, if you put all Class D tenants in a Class A 4-plex, what do you think will happen?

If  you are in the 20% range, you are better off going to the casino and putting all of your money on black and seeing what happnes.Since you are out of the area properly calculating cash on cash returns can be difficult because you have variable expenses to calculate. 

I am looking for a 4-plex in the area and I have a difficult time finding good deals.

Cons - directing tri plex cash flow away from other investments for 4 years but also not impacting my W2 lifestyle.