Last time this came up, I pointed out that prior to CK2, the most successful Pdox game was HoI3 (or seemed to be). Every new game since has been massively more successful than its predecessor - CK2 achieved a lot of fame and has a huge amount of popular YTers playing it. EU4 carried some of this success forwards, and has sold comparably, although it's been out a shorter time so it's not sold as much in absolute terms. If Hearts of Iron 4 carries this combined momentum with it, it could definitely overtake CK2 and EU4, since World War II is a historical period that more people are interested in.
I would also contend that HoI4 looks like it's shaping up to be one of the most accessible entries, and games seem like they will take far shorter amounts of time to come to a conclusion. You could plausibly start and finish a game of HoI4 in one session (single player), they cut down on micro and the battle planner stuff means that you'll be spending less time pausing or running at slow speeds for average players. That is all to say, I think it could be easier for more people to get into it, with less of a steep learning curve. Thus more successful.
