Yes, 4% is appropriate as we are talking about owner occupied houses. E.g., I have 50% equity in the house I live in and decide to leverage my portfolio and take out 20% equity to buy shares. A bank will offer me a sub 4% fixed rate mortgage on that (perhaps depending on age etc). They will probably do this even if I just tell them I want a flash car and a holiday, let alone something worthy like buying shares.

You still seem to think banks don't offer margin lending - if I walk into ASB and ask for a loan based on my shares as collateral, they will say 'sure, what do you have?'. The amounts they will loan against different holdings are published here

https://www.asb.co.nz/documents/asb-...ding-documents