我有一种解法

1. 取最小日期和最大日期值，生成数据集，处于这两个日期间的每一天(含端点)一条观测；
2. 上一部中生成的数据集与你给出的HAVE数据集作全连接，去掉未用药的记录；
3. 使用汇总函数对每个受试者每个日期进行计数，计数为4的为使用了4次药物；

注： 若要知道的是使用了4种药物的受试者，应该在第3步前进行去重。

简单点可以这么处理一下

DATA HAVE;
INPUT ID $  BEGINDT MMDDYY10.+1 ENDDT MMDDYY10. DRUG $;
FORMAT BEGINDT ENDDT MMDDYY10.;
CARDS;
1 01/01/2017 01/10/2017 A
1 02/15/2017 03/15/2017 A
1 04/01/2017 04/10/2017 B
1 05/01/2017 05/02/2017 A
1 06/01/2017 07/31/2017 B
1 06/15/2017 06/26/2017 C
1 07/01/2017 07/02/2017 A
1 07/19/2017 07/20/2017 D
1 08/01/2017 09/10/2017 A
2 05/02/2017 06/01/2017 B
2 06/10/2017 07/10/2017 B
2 06/10/2017 07/10/2017 B
2 08/16/2017 09/15/2017 C
3 11/11/2016 11/16/2016 A
3 11/12/2016 11/15/2016 A
3 11/12/2016 11/15/2016 B
3 11/15/2016 12/15/2016 C
3 12/02/2016 12/03/2016 D
;
RUN;

PROC SORT DATA=HAVE;
BY ID BEGINDT;
RUN;
DATA HAVE1;
SET HAVE;
BY ID BEGINDT;
DO I = BEGINDT TO ENDDT;
OUTPUT;
END;
KEEP ID DATE;
FORMAT DATE MMDDYY10.;
RUN;

PROC SORT DATA=HAVE1 NODUP;
BY _ALL_;
RUN;

PROC SQL;
CREATE TABLE HAVE2 AS
SELECT A.ID,
       A.DATE,
           B.DRUG
FROM HAVE1 AS A LEFT JOIN HAVE AS B ON A.ID = B.ID AND A.DATE BETWEEN B.BEGINDT AND B.ENDDT
;
QUIT;

PROC SQL;
CREATE TABLE FINAL AS
SELECT ID,
       DATE,
       COUNT(DISTINCT DRUG) AS DRUGCNT
FROM HAVE2
GROUP BY 1,2
ORDER BY 3 DESC
;
QUIT;


感谢上面两位的指导，我大概就是按照你们的思路写的，谢谢