Prepare pasta to directions on the box. Cook 1 minute less to make al dente.
In separate 2 qt. pan melt butter on medium heat. Stir in flour and whisk constantly to blend with melted butter. When flour is completely included, slowly stir in the milk while whisking all the time. Make sure there are no clumps & everything is becoming silky smooth.
After milk is totally incorporated add the mascarpone, parmesan, gruyere & white cheddar while stirring to maintain consistency. Reduce heat to low. Add black pepper & nutmeg. Stir for 2 more minutes then turn off heat.
Add drained pasta & stir to coat completely.
Empty pasta into 9 X 12 dish.
Combine the panko breadcrumbs with the melted butter and smoked paprika. Then mix in evenly the sharp cheddar. Spread the mixture evenly over the top & bake at 350° for 25 minutes.