How can I use substring to extract the four last characters of a column if I do not know the number of characters that are within a field

Question

I want to only get the last four digits of a column using substring unless there is another way to do so.

Here is what I have so far:

Column fieldColumn = dataset.apply(fieldName);

fieldColumn = fieldColumn.substr(functions.length(col(fieldName)),functions.length(col(fieldName)));