Sh4 ships stay on course? That's news to me
SH4 has much better merchant behaviour, because they will actually sail away from threats that they detect, while also zig-zagging. SH3 they stay on the same general heading, but make regular zig-zags. They do not try to "run away" from you like in SH4.
What I think might be happening is that whatever mod/version of SH3 you're using has more "sensitive" merchants that see missed torpedoes (or your periscope) more easily than whatever version of SH4 that you used. Ships in both SH3 and SH4 will only start evading when they see the torpedoes (or you, or another enemy unit). Most mods have it configured so that they keep zig-zagging for about 15 minutes after last seeing a threat, then they go back to "normal". In SH3, they never change their general heading or speed (except to evade torpedoes that are actually coming at them); in SH4, they will turn back to their previous waypoint after the threat is gone (which sometimes causes them to sail right back towards you).