I tried either HOI 1 or 2, I forget which. Played a fair amount of HOI 3, and also rage quit and equal number of games. I agree completely with Kevin. HOI 3 is just way more accessible than the other version, in everything from the innovative battle line system. Where you draw a frontline and arrows to attack an objective. To a still deep research system, but not ridiculously complicated research system like HOI 3,
There are lot of quality of life improvements, including not being forced to spend 3 hours every game reorganizing the OOB for the major countries, and taking a second job as Head of Army personal that was required for HOI 3.
HOI IV has some beautiful executed sub-system like a production system, which is probably the best production system ever done for any game ever.