Save for down payment vs borrow against equity?

[Jerry Baldwin]

I have a two family house and the plan is to buy another one in 4-5 years. Keeping the current one, rent out both units, and move into the new one. We've been saving up for a down payment to do 20% down. Currently have about 40k in the bank and aiming to have 60k for a down payment with more left over. 

My wife and I are currently not maximizing our Roth IRAs in order to save up. We're in our early 30s and putting in about 3k per year for each of us. If we double that to maximize contributions, we'd have that much less for the down payment in 5 years. We're currently saving maybe around 1k per month, but that may change. Current plans to buy a new (used) car later this year and also have another child, so she'll be out of work for a little while.  

I could possibly borrow up to 30-50k or so against our home equity, most likely through refinancing our mortgage, which originated a few years ago at 4.25%. Would it be better to put more into our Roth's for retirement and borrow more for the down payment (either soon to take advantage of low rates or in a few years, closer to the next purchase) or continue saving and putting that into a HYSA, CD's, maybe a little investing?