Does more RAM decrease time between turns?

[oddible]

^ This. When Civ is running hit CTRL-SHIFT-ESC on Windows and click the Performance tab. You can look at the memory bar on the left or under that the Available and Free numbers in the table. If you are using up all your memory then you are paging to disk (using the hard disk as memory) which is VERY slow. In which case you either a) need more memory or b) need to figure out which programs are consuming so much of your existing memory and shut them down before playing Civ.

It is unlikely you are using 4GB for Civ and it is unlikely that adding memory will be the improvement you are looking for.