Dump cake with white chocolate

This is a possible substitution, you could make with these factors in mind:
The original recipe had 4 oz unsweetened chocolate, which has almost no sugar.
White chocolate has about half an oz sugar per oz candy/coating, so using 4 oz will add 2 oz sugar to the recipe.
So be prepared for sweeter taste and/or reduce the other sugar amount called for in the recipe.
Last, white chocolate has no chocolate solids and so will not have the same bitter/umami taste of unsweetened chocolate.