Depends on your taste. In my opinion Sh4 is the best of the franquise, but it lacks all the tons of quality mods available for SH3, which set this one in a different level. Hence, with SH3 you never can go wrong.
For Sh4 you have however the excellent Operation Monsoon and yes it has a new environmental mod that makes substantial changes to the water etc. If you get hooked by it, the game itself is more refined and plays better on the same PC, with faster loading times and better use of time compression without bugs.
Give it a try, you can't lose anything.
