[Exim] Exim 3 vs Exim 4...does it matter to a newbie?

Somebody made a comment in another thread that Exim 3 and 4
are VERY different.

I am about to learn Exim. Got the O'Reilly book right here next to me....
Exim: The Mail Transfer Agent. Looks like it is based on Exim 3.

Did I waste my money on the book?

Should I learn Exim 3 anyway?

Are the fundamentals the same? (important question to me)

Is Exim 4 very different because of advanced features?

Any thoughts would be appreciated.

Andy