Is attaining debt at a 4% rate mortgage riskier than utilizing their float?
"Don't use leverage" is good advice for people who do not have the skills and experience to properly manage risk (which is to say, over 90% of the population). The proper answer is something like a long explanation of the Kelly Criterion, risk of ruin, how to evaluate the very fuzzy notion of your own career's job security and social safety net, and a ton of other factors.
Circling back to your question about a mortgage, though - just don't become "house poor". You want to have free cash flow above monthly expenses. Outside of that, on a 30-year fixed-rate mortgage basis, feel free to borrow and invest literally every dollar the bank approves you for. Especially if the loan is single-action or your investments get funneled into creditor-protected retirement accounts.
That is a very interesting strategy. Do you have any recommendations for books or other resources that discuss this?
If you don't pay your mortgage, it's generally either impossible or not worth suing you for the outstanding balance. Usually what happens is the bank spends a few months going through the foreclosure process, trashes your credit score, takes your house, and evicts you. This is bad, but if you owe $400k on a house that's worth $300k, it's less bad than continuing to pay your mortgage and dump an extra $100k down the drain. You save up first + last + security + moving costs for an apartment or renting a house out of the freed up cash flow, deal with the fact that getting credit is going to be difficult for a while, and move on with your life.
I know, citation needed...
Edit: I guess a secondary concern is the risk of interest rates going up, will you be ok if the cost of servicing the debt doubles?
Are people seriously considering borrowing against adjustable-rate instruments to buy equities?
My interest is that the policy risk according to Berkshire actions is less than 4% debt they could likely attain (or better). In other words - they are recommending investors not to use leverage, but in a sense they do use it against policy risk.
The trade off between debt and policy risk is interesting as both greatly increase returns by leverage.