Staying with the Atlantic in SH4 would mean doing Operation Monsun from lurker_hlb3, from years back, or Dark Waters, which is a re-worked OM from Fifi. Both attempt to emulate a modded SH3, and include the original SH4 version of Op Monsun, along with the Med, and other areas. Then there are several offerings from the KSD team and assorted fellows, of which I am not too familiar with, so we'll have to await other's comments. There is nothing though, that can give you the h.sie / Stiebler "hardcore" experience available in SH4, and it does not have the same "claustrophobic" atmosphere of SH3. It also is not as pretty as SH5 - however, it is a better platform than SH3, and not as restricted as SH5 in modding certain aspects, such as adding player subs.
Advantages of SH4 over SH3 is a "dynamic loading" of the campaign files. In SH3, there are 3 campaign files, and they are huge, fully-loading at the start of the game session. The larger the mod, the longer the loading time. The entire Roster, all ships, planes and land units, are loaded into the game. In SH4 though, the campaign is done in "layers", and the game only loads what is in the immediate area. Nothing spawns into the game until it is almost close enough to be seen. The "assets" are tracked in a database, so it looks like the world is alive all around you, but only the immediate area is actually 3D. This reduces load times considerably, and makes the Save game much more stable - so long as the modders (
) don't make things a mess with bad data...
Disadvantages of SH4 are that the devs never quite finished the game (surprise there, eh??), in spite of the "payware" v1.5 "update" that added the U-Boats to SH4. Equipment "upgrades" can be bothersome, and quite a few do fail, especially turm upgrades in OM and DW. I am not certain how deeply the KSD folks got into the weeds with atttempts to "fix" the issues, but surely someone can comment about that. There are work-arounds for the issues, but like a lot of thngs "Silent Hunter", are kludges...
Since you have it, I would load it up, add Dark Waters to it, use MultiSH4 and change the Save folder name, and start experimenting. Run a career from a base / timeframe of your choosing, and do the same in SH3 at the same time. Compare your experience between the two. I like all three games for varying reasons, but for me, it is still SH3 for the U-Boats, and SH4 for the US Pacific, with SH5 (with TWoS) in a distant third...
