I finally got around to buying and start playing SH4 last week.
I've spent several years playing SH3 off and on, with the GWX mod really enhancing that game for me.
I was distracted from that game here and there by playing other games (RPGs like Skyrim, Mass Effect, Mass Effect2, et), a hard drive crash and loosing SH3 game disk having to buy it again, etc.
So there I was with SH4 and learning to play it. I must say that it is quite different than SH3 in many ways.
I won't say that I hate the interface, but it has taken me quite a bit to get used to it. And it seems a bit harder to sink ships (took 5 torps to sink a small composit merchant last night....all solid hits on it, but it just didn't want to sink).
But if I have to complain about something, it would be the volume level of the voice actors. I have to turn the music and all the other sound effects way down to hear my crew.
On the other hand I do so love having to remember to send the TDC information to my torps before shooting. Was hilarious the first time I played using the tutorials and could not figure out at first why all my torps were going the wrong way!
I won't say that I like one game over the other, especially since I just started SH4 and have played SH3 for a very long time now. I'll wait until I have a lot more cruise time on SH4 under my belt.