The report plainly states:
> When faced with a hypothetical expense of only $400, 59 percent of adults in 2017 say they could easily cover it, using entirely cash, savings, or a credit card paid off at the next statement (referred to, altogether, as "cash or its equivalent") https://www.federalreserve.gov/publications/2018-economic-we...
The 4 in 10 comes from the very next paragraph of the report:
> Among the remaining 4 in 10 adults who would have more difficulty covering such an expense... https://www.federalreserve.gov/publications/2018-economic-we...
and, from the very next figure in the report, none of the 4 in 10 would use "cash or its equivalent" to cover such an expense. https://www.federalreserve.gov/publications/2018-economic-we...
From this data, it is reasonable to conclude that "Four in 10 adults would not use 'cash, savings, or a credit card paid off at the next statement' to cover such an expense."