First of all, using a loan as a down-payment is something banks frown on. You may want the money out for some seasoning time, so that the bank will consider that your money.
Second, using loan as a down-payment gives you a lot of risks, as you are leveraging huge (the fundamental reason why banks do not like it). Cash flow won't be great, factoring in the interest rate. So you need to rely on appreciation. If the property depreciates, you are in a big trouble. A gentle depreciation is expected by most in the coming 1-2 years. So maybe you need to give it a second thought.
If you still want to go any how (yeah, you can still go, higher risk leads to higher return, though it is a big bet), the cash-out decision depends on your situation. If you need the cash right away, you can choose the conventional cash-out refi (I don't think you can get as low as 4%) or Home Equity Loan (HEL). If you do not know when you will need the money, go for HELOC, which works more or less like a big credit card, usually with a variable rate (some banks allow you to lock when you take a big chunk out at a slightly higher rate).