Seems like you're asking two separate questions...first, is borrowing (or leaving financing place) at a long term fixed rate a good strategy right now? I think YES. If the long term rate of inflation is 2-3% (and the Fed has taken extraordinary measures in recent times to try and induce that), and you're locked in at 4.0%, you're borrowing money at an effective rate of 1-2%. If you believe (as you seem to, and I do as well), that we'll see higher than targeted inflation over the next several years, then it seems like a no-brainer to leave in place your fixed rate financing, as you'll be borrowing at or below the rate of inflation for some period (free money!). All while enjoying rising rental rates and property values.
Then, if I'm the hypothetical guy with the $750k, and I've decided (as above) that I like the long term fixed rate leverage play, then I have to ask myself, do I continue to allocate dollars to RE investments, or do I diversify?
I have an undergrad degree in finance and worked in the mutual fund industry earlier in my career. Now I'm in real estate (day job and personal investing) and my wife is a financial advisor. We have some interesting "conversations" about what we'd do with my 401k rollover if I leave my day job. Even with (actually, probably DUE TO) my earlier experience with the stock market I'm all about a checkbook SDIRA....she'd love to get it all into the stock market but the thought of that literally makes me sick to my stomach!