Using the float shares the solvency risk with borrowing, but they aren't paying 4% on the money.
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.