the count starts with 0, so it would be the second "s"
EDIT: If you like to work without separators, you can use the "left" and "right" expressions.
Based on your example:
wwwbbbss
to get the 4th entry, use this code:
right(left("wwwbbbss",4),1)
this would return the first "b"
EDIT2: mid can be used too:
Same example as above but the expression would be:
mid("wwwbbbss", 4, 1)