这个在sas tutorl里有说明 就是重新定义之后 只会显示之前的脚注 所以footnote3 和4 不显示

When a FOOTNOTE statement is specified for a given line( with a number as suffix), it cancels the previous FOOTNOTE statement for that line and for all footnote lines with higher numbers.

Redefining a title or footnote line cancels any higher-numbered title or footnote lines, respectively. In the example below, defining a title for line 2 in the second report automatically cancels title line 3.
详见：sas tutor base之 creating list reports 的37页

When a FOOTNOTE statement is specified for a given line, it cancels the previous FOOTNOTE statement for that line and for all footnote lines with higher numbers.
From SAS Help

受教了。