Let’s first find out how much work they can do together in one day:

A’s one day work = 1/15 B’s one day work = 1/20

Together, their one day work = 1/15 + 1/20 = (4/60) + (3/60) = 7/60

So, in 4 days, their total work = (7/60) x 4 = 7/15

This means that the fraction of work left is:

1 - 7/15 = 8/15

Therefore, the fraction of work that is left is 8/15.