Freeze issues with rain catchment systems, Zone 4

I am planning to go with rain catchment for my complete water supply on my home build.  My house will include a bermed north side and I plan to install the tanks inside the berm.  The roof will be slopped to the north so the tanks will be on the best side of the building.  My biggest question is tank freeze.  Will a bermed side of the hill with 3 feet or dirt surrounding the tank/s on all sides be enough to keep them from freezing or would they need to go below grade?  Will placing the tanks very close to the outside of the north wall help to keep them from freezing?  Two more random questions. What type of pump do you use for your water pressure? Is the standard first flush system using only a 4" collection pipe enough to flush a 1500 sq/ft roof?

In S. Colorado, many folks here will "half-bury" the tank, build sides up with their choice of material, and build roof system ... this is for a large tank of 1000's of gallons, which in itself offers a bit of freeze protection for this area (too much water to freeze solid). The more you berm, or insulate w/ rigid sheets, the better.

Rigid insulation might become a highway for termites, or campsite for ants, so protect per your region (or use something different).

We use a grundfos water pump, w/ small internal reservoir, and this gives us about 40psi from our IBC (in a utility shed) to the rest of the system; it takes some power to get spinning, so might need a "kickstart" device in front of it.

Hope this helps ...

4 inch pipe is ok for your first flush unit. Experiment with the length required to have clean water flowing to the tank.
Burying the tank and the pipes is certainly practical, but make sure you dont push the tank walls in.
Think about that point and ask around.
Just about any pump will work, I use 12/24V RV pumps and a Grunfos separately.
I would recommend a 20,000L tank

I was planning to go with 3 1500-1700 gallon tanks to keep the size down since these will be buried in my berm.  Are there any cisterns that are meant to withstand pressures from being buried?

underground tanks or septic tank bodies

Ask the manufacturer. They will be clear about tanks that can be buried vs. tanks that cannot. The engineering is substantially different.