In a large bowl, cut the flour into the shortening. Add the yolks, mix well. Add the baking powder, 2T sugar, and citrus juice. Mix well; the dough should be crumbly.
Grease the bottom and sides of a 10x15 jelly roll pan (or a 9x13 baking dish). Line the bottom with the dough, reserving some for the top. Spread the jelly over the dough.
Beat the 4 egg whites with 1 cup sugar until the whites are fluffy. (Add in the walnuts at this time, but just fold them in — don't beat the walnuts!) Spread the egg white mixture over the top of the jelly. Crumble the remaining dough on top of the egg whites (crumble the dough such that very little, if any, white is showing through).
Bake at 350F for 1/2 hour, or until brown. Cool. Cut into squares.
(This works well as a coffee-table munchy, or as a slightly elegant dessert.)