the doom of the ring??

[Eruwen]

would there be a dark lord in the 4th age too?

Well, it depends on whether someone very powerful switches to the "dark side", if I may say it in that way. The first Dark lord, Melkor, did that. I think the only reason there was another dark lord after him was because Sauron started out kind of like Melkor's apprentice.

Sauron learned from Melkor and thus decided to take over after Melkor was defeated. Now Saruman would have been the likely choice to be the next dark lord if it hadn't of been for him getting killed. So it's kind of up in arms at who would be the next.

But think of this, no world's perfect. Sadly, not even Middle Earth is. There always was someone wanting to screw everything up, and there always will be. That's just how it is.