Patent Trolling or IP Defense

Imagine four different situations.

1. A company that owns a patent for some period of time (months or years) discovers that another company begins to infringe on that patent.
2. A company that has owned a patent for some period of time (months or years) discovers that another company has been infringing on that patent, possibly also for some period of time in months or years.
3. A company purchases a patent (along with the originating company or perhaps in an independent sale).  They later discover that another company begins to infringe on that patent.
4. A company purchases a patent (along with the originating company or perhaps in an independent sale).  They discover that another company has been infringing on that patent, possibly also for some period of time in months or years.

Which of these merits a lawsuit?  When you look at them together, they don’t seem very different.  In all cases, one company own the IP and the other is using it without permission. Under current law, all of the IP owners have the right to sue for damages and have grounds for that suit.

But we often see companies in the fourth category referred to as a patent troll.  Many pundits suggest that the law should be changed so that these companies do not have the legal right to sue (or at least to win a lawsuit). 

I know all of the lawyers out there will have opinions on the legality of these situations.  But I wonder what the rest of you (us) think about separating the fourth category as something unique and with different legal rights.