It seems like I opened up Pandora's Box and that I oversimplified the limitations of an exe file versus the various OS that Paradox fans have access to when they are enjoying the different strategy games out there. Sorry about the oversimplification! I am not a tech person nor do I have the background to know whether the RAM bottleneck is OS based, exe based or some combination. I'll read this forum thread and learn from those of you who do know (and thanks for your replies btw).
I simply wanted to understand how HOI4 was built - 32 bit versus 64 bit, and how much RAM the exe file will allow access to (if the OS is not the limiting factor). For myself, I am running an Intel i7-4710MQ CPU @ 2.50GHz, 2501 Mhz, 4 Core(s), 8 Logical Processor(s), a Titan video card and 16 GB of RAM in a Windows 8 environment, so I believe that my CPU and processor can handle unmodded HOI3/HOI4 + any DLC that comes out -- and if HOI has an AI that needs more horses than that - I would like to challenge it to a duel over Europe...

.
I enjoy HOI3 tfh as well as various mods, some of which seem to take RAM use beyond 3.5 GB of RAM (according to my resource monitor when I get a CTD crash or an outright crash). I don't know if there is any exe solution for enhancing RAM use while playing heavily modded HOI3 tfh (beyond Podcat's 32 bit exe for HOI3 tfh - thank you for that Podcat!). Hitting the RAM 'wall' prompted my questions for HOI4.
I enjoy the HOI series enough that I wanted to understand what level of system resources basic, unmodded, pre-DLC HOI4 needed to run. To part of my original question, does it use, for ex., 1 GB of RAM when the game reaches 1943? Any modding that we do to HOI4 will add to the RAM load (again - not a systems expert so I don't know how much the load will increase). If HOI4 was already using 3 GB of RAM in mid-game, then that would appear to offer limited room to grow where modding would add to RAM use. If RAM use is at 1 GB in mid-game, then modding would appear to have significant room to grow without hitting the RAM ceiling.
I thought that the above level of background was too much for my initial question but I am including it here to help those reading/responding better understand the 'why' behind my questions.
Thanks again for those reading/taking the time to respond to my questions!