Much like the lavish parties, elaborate gowns, and decadent culinary spreads that are so crucial to “The Gilded Age,” subtlety is not a commodity Julian Fellowes’ historical drama has ever traded in. Its dedication to the grandiose, while delivering character-driven storylines, is core to the appeal that has seen the HBO series go from strength to strength. The fourth season continues to raise the high-society stakes in satisfying fashion, reign…