Alot of people love to speculate about the 4th being the main evil villian of the series....but really think about how the context of that situation affect the whole storyline...

As it stands right now, the 4th is thought of as one of the greatest heroes of the past. Any time a comparison is made between the 4th and Naruto, it is in relation to something Naruto has accomplished that would hold him in high esteem. The very notion is synonymous with greatness.

For the 4th to become the greatest villian would completely tarnish this perception and would imply that, maybe such a path that Naruto follows does not in fact lead to that of a respected, selfless leader afterall.

Lets take Star Wars for example....
What significance does Darth Vader's role play in the development of Luke? Here, like Naruto, we have a situation where a promising individual emerges, whose past has been hidden to all including himself, and by revealing the truth of his father, the context of Luke's role within the story is to overcome his destiny of falling down the path of corruption that his father took.

On one hand, you've got an individual that strives for the perfection laid out by his predecessor, while the other you've got one that strives to oppose it.

In other words...it would be stupid to make the 4th a villian.