Is LR4 worth purchasing if I already have Photoshop CS 5.5?
I don't need the anti malware software and would likely use the Sony software rarely.
LR4 is worth it if you would like to step up your workflow. You can do the same editing in LR as in PS, but the ease of managing your library is much better with LR4 and switching between photos/tasks.
I suggest you give LR a try before you buy if you are on the fence.