If Hearts of Iron IV generated the same gross revenue on the Epic Games Store, Paradox Development Studio would keep approximately $256,048,200 - that's $26,386,200 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $34,779,300. Epic charges 0% on the first $1 million in annual per-app revenue and then 12% on the remaining amount. On GOG (30% cut), the developer would earn approximately $203,579,250.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.