Is primary residence HELOC interest tax deductible?

[Kin Meng Sio]

Hi,

I live in Washington state and I have an in-state 4-plex currently under contract with conventional financing. I plan to use one of the following options to partially fund my down payment + closing cost:

1. HELOC from my primary residence

2. 401K loan

I'll be paying back to either loan in couple of months. I'm debating which fund I should use in terms of tax efficiency + opportunity cost. Couple thoughts:

1. If I take money out from 401K, i lose some compound interest until I pay back. 

2. I'm using after-tax $$$ to pay back the 401K + interest. 

3. However, I'm paying back to myself with the interest rate. 

4. HELOC interest is lower and is interest-only payment, so I have more flexibility on my personal cash flow.

5. Not sure if HELOC interest in this case is tax deductible or not. It's a primary residence HELOC but it's paying against a investment property.

Any advice? I'm searching on Google regarding HELOC interest tax deduction but I haven't found anything specific to my case yet.