(Word of warning, this will be a bit... lengthy, as there is... a lot of ground to cover...
you have been warned.
)
As propbeanie pointed out, there are numerous choices when it comes to modding up SH4... a lot, but... will only focus on the 2 main ones here. Seeing as how you mentioned of wanting to keep to a certain feel of the game, as you have in SH3... am guessing you mean with staying in the Uboat realm of things... then will focus primarily on that side of the equation.
Of the Uboat side, there are 4 that come to mind here...
1. Original Operation Monsun
AfaIko, this requires v1.5 of SH4 (which by your comment, you should have what with mentioning the 'Gold' version, on cd/dvd available...
)
This mod set, requires a bit of work to set up &... making sure to grab the right mods to add in, to round it out. (this is however, included in the next mod set mentioned...) That said, thee are add in mods, to avoid altogether... however.
2. Oper. Monsun: Dark Waters (latest v6)
This mod set, is the original, cleaned up & refined with some of the additional mods, plus some further refinements, that got their start with SH3 & ported over for SH4.
This, by far, of the 1st 2, is the one to focus on more... in My humble opine... if you just want to focus on this set up option. If not, then onwards we go...
3. KSD II: The original.
This particular mod set, is like O. M.: Dark, a further refinement of the Uboat war... only downside to it... it was originally set up, to accommodate multiplay with all involved... having the same mod set up. In its own right, not really a grand choice... unless you're willing to find others with this set up, to multi play it... which.. not sure if there are or not, others still doing so... To be quite completely, frank about it.
That said, turning to the final option...
3 KSD II: Ace edition.
As with O.M. : Dark, this set up, is a further refinement in some volunteers, took time to iron out translating the orders, messages & such... into an eng. translation. While not perfect... it does make it to where one can roll with it & do a career, which the original didn't.
Those mentioned here, so far... are just the Uboat side of SH4... & 4, does have strengths as well as weaknesses, as outlined by propbeanie in their reply.
The biggest strength is, unlike in 3... you don't get a patrol lasting 24 hours, then left to 1's own devices on what to do, short of hitting up a resupply ship or milk cow uboat... which unless you have it set up to simulate aka faux docking, which was in My opinion... a buzz killer.
4, however... fixed it, to where you can pull, in to a friendly port, restock back up then head back out fairly & relatively pain free... & continue on your merry way... sinking targets.
Downside to that, still... is that any major damages to your sub... have to end up in being repaired, after docking back in at base. As was the case, in real life...
Patrols, can be extended as did happen in real life, by doing a status report... aka radioing home. Where you then get new orders.
Just as with 3, in 4... to log your kills, you'll still have to do a RTB, to register them. Thereby, ending that patrol.
The damage models, are a bit better in 4, over its predecessor, 3... which is, a very big + factor. Some mod sets, have further refinements in that area... which makes it all that much more enjoyable, when you see or watch a target go under. Just be careful, to not get star struck on watching all the time, as the defenders, can be... madder than hornets, out for your head to mount on their trophy wall...
Which brings up 1 final downside... the A.I., as it goes for the enemies, can either be... dumb as a box of rocks, or downright evil nasty... depending... on how the modder, set them up to be... as that are, is still a bit of a work in progress, to refine them... to be more... realistic, in their reactions & in their abilities. Though, even 3... had that as well.
Hope this helps... as always...
M. M.