Hi and welcome aboard!
The first thing to consider before looking at mods is exactly what you want from the game. Do you want to play with U-boats or with US Fleet Boats? If the former then you need Operation Monsun, which gives an SH3-like experience to your U-boat campaign. If you decide on that then there are several add-ons that go with it. If it's fleet boats then cdrsubron7's advice is spot-on. GFO fixes most of the bugs in SH4 while still leaving you playing the stock game.
If you want to try both the first thing you need is
Multi-SH4. This little program lets you run several different games at the same time, making it easy to run both a U-boat and a Fleet Boat campaign, or try any or all of the Pacific campaigns against each other. No matter what you do Multi-SH4 is a good thing to have.
Any more than that can wait until you've decided what you want.