Re: one wt over another
A 5wt rod will be a bit more powerful than a 4wt rod, on average. It should have more pulling power. The 5wt line will work a bit better than a 4wt line at turning over larger flies and weight. It will not present flies quite as delicately as the 4wt though.
I'm fairly sure most 4wt and 5wt lines from the same manufacturer are built on the same core, which is what determines the strength of the line.