data bbb;
set aaa;
by id no;
retain y;
if first.id then do;
        if x>=4 then y=1; else y=0;
end;
if lag(y) = 0 and x>=4 then y=1;
run;

结果不对!
x=4 应该y=0

没注意，4的那一条是0哈。最傻瓜方法，俺现在逻辑对4那条再单独处理一遍就OK了

data bbb;
set aaa;
by id no;
if first.id then flag=0;
if x=4 then flag+1;
if flag=0 or (x=4 and flag=1) then k=0;
else if flag>=1 then k=1;
run;