The 3rd generation isn't that old. What would the 4th generation add, and are there even any rumors that a 4th generation is coming? I mean, it certainly will happen eventually, but there has to be a reason to bring out a new generation, and currently I don't see anything that would cause that to happen. Of course, I might be missing something, so correct me if this information is already out there.
Should you wait? You can always wait and there will always be a new generation coming. Buy what you need when you need it. I would go 15GB minimum, but you might also not need much more. I currently have around 2100 songs on my 20GB iPod (2nd gen). All songs are recorded at 256kb or higher, most at 100% quality VBR. I am using about 12GB. That is almost my complete CD library, so I think it would be hard for me to go over 15GB anytime soon, but I'm already using more than 10GB.
One thing that will influence the above is that the iPod is usable as a mobile hard drive. If you plan on sticking files on your iPod in addition to songs, factor that into your size needs.