How clever, but unfortunately it doesn't solve my problem.
All the cells need to be open for the user to fill-in values. It is an area
of four cells, where the user has to fill in a distribution of 100 percent.
It is allowed to fill in one, two, three or four cells, but the sum should
add to 100.
What I like to do is to allow the user to fill in values in the four cells,
then validate the sum and don't allow the user to move to the next step until
the rule is satisfied.