Much will depend on what particular functions you want to improve. Where SSDs do help is with searching through libraries, other aspects like importing/exporting files, building previews or the responsiveness of the develop controls really won't see much(if any) benefit from using SSDs.
I am currently looking at my drive situation and wondering where the best bang for buck is going to be regarding speed.
Right now the best place to spend money on improving LR4's speed is a faster CPU.
I've been down the route of putting the library & ACR cache on an SSD, then putting the OS+programs onto an SSD, but apart from faster program loading times, the operational speed of LR hasn't been as dramatically improved as you might expect from reading about this issue.
The only time I've seen a dramatic
improvement from an upgrade was moving from a 4 core 2.4ghz CPU to a 3.7ghxz 8 core system.