The hybrids only have a small SSD-section, so only the apps you've recently used a lot will be stored in that part of the drive. The rest won't be much faster than regular 7200RPM speeds. An SSD however will give you top speed with everything you do. SSDs are also completely silent, where most hybrids are not. There's even a lot of talk about an increase in volume and vibrations. It'll depend on the drive, and on whether you care about sounds, but it's something you should look into first.
You're already at 4GB RAM. Unless your using a lot of really power hungry applications and you're sure that they will actually utilize more RAM, I'm guessing that at this point 4GB is enough. If you really want a speed increase you should go for the SSD. The advantage is that even if your MacBook runs out of RAM and starts swapping, the swap file on the SSD can be accessed much faster and it'll feel like you're just using RAM anyways. Tests will show you that SSD swapping isn't as fast as real RAM obviously, but in the end it's about how you experience it.
You can also buy a smaller SSD and use it as a boot drive, remove the optical drive and replace it with an OptiBay (hard drive caddy) in which you can install the SDD. Or put the SSD in the place of the current HD and move the HD to the place of the optical drive. That way you'll have the SSD speed for OS X and your applications, and the stock drive for storage. That is of course if you don't use the optical drive that often and you can miss it (or put it in a USB enclosure so you can still use it). I do believe this (removing the optical drive and replacing it with a HD) will void warranty though, but I'm not sure.