For comparative purposes, consider that in the 1960s the discount rate was 4+% and the mortgage rate was 5.5-5.75%.
In terms of the big picture (what to do) - I have no idea, but I'm leaning to borrowing a lot and locking in a long rate of 4% being smart. This is a pro-inflation view (meaning, I think inflation is likely), that might be wrong, so YMMV. In a deflationary crisis, a big mortgage at 4% when incomes are falling consistently could be a terrible move - IF you think they really will make you pay.
inflation has generally trended with interest rates, but it doesn’t have to be so, especially if printing money is the cause of the inflation,
I think.