Keith_UK:
With SH4 v1.4 you should have downloaded this:
http://files.filefront.com/RSRDC+V35.../fileinfo.html
this
http://files.filefront.com/RSRDC+V35.../fileinfo.html
and this
http://files.filefront.com/RSRDC+SH4.../fileinfo.html
But be aware it's no longer supported or being developed for v1.4, only for v1.5.
I'm sure you can find more help in the RSRD thread here:
http://www.subsim.com/radioroom/showthread.php?t=123467
Mainexpress:
You should grab the U-boat addon (1.5) if you can since most mods are starting to require it. Without it you may be stuck running older versions of the most popular mods. There is no harm in having it installed if you don't use it, as far as I know it doesn't touch the U.S. careers at all.
As for the most popular mods, they are all stickied to the top of the Mods Workshop forum here :
http://www.subsim.com/radioroom/forumdisplay.php?f=219
Most people run a combination of:
-ROW Pacific Environment (graphics improvements, better seas, skies, clouds, etc)
-Run Silent Run Deep (completely reworked campaign, missions, etc)
and either "Trigger Maru Overhauled" or "Real Fleet Boat" for everything else, depending of which of them you like best.
After installing these, SH4 is like a brand new game, light years beyond default.