WELCOME ABOARD!
SH4 has the U-Boats Add-On which, with Operation Monsun installed, becomes a full-blown Atlantic campaign. It lacks some of the cooler immersion mods available for SH3, but it is much the same game except with several improvements. I don't currently play the Pacific campaign either, being of the obsessive-compulsive persuasion and having to play in a linear fashion. If I ever reach 1942 I'll be playing in both theaters.
SH3? I also play in a straight line. If I die, or retire, in August 1942, my new career starts in September 1942. I may be starting a novice career in 1944 but sooner or later I will finally reach the end of the war. Or so I keep telling myself.