If the same employees appear in all files then ignore files 1,2,3 and just get the data from file 4.
But assuming that different employees exist in each file then you could load up all of the files individually then add a step to combine them into the single table like this
select ename, sal, job, dept
into tbl_emp
from file4
union
select ename, sal, job, null from File3
union
select ename, sal, null, null from File2
union
select ename, null, null, null from File1
Add a final step to get rid of the "temporary" tables that you used for each file.
Note I started with the file that had all of the columns in it (File 4) rather than with File 1 - this means I didn't have to define a column type for the null columns when I was initially creating the table.
If the table
tbl_emp
already exists then it doesn't matter what the order is and you need to move the insert like this
insert into tbl_emp
select ename, sal, job, dept
from file4
union
select ename, sal, job, null from File3
union
select ename, sal, null, null from File2
union
select ename, null, null, null from File1