Dan wrote:

If it is as crucial as I have been led to believe that the line weight
matches the rod weight, what weight line does one use if a rod described as
a "4/5" weight?

FFing ain't brain surgery; there's not a lot about it that is
'crucial'.

Still, one common answer to your question (and one a number of rod
makers advise) is "either a DT4 or a WF5". In truth, most rods will
cast well--or reasonably well--more than a single line weight. A rod
rated 4/5, *if* correctly rated (some aren't), would probably cast
either a 4-weight or a 5-weight line fairly well. It is quite possible,
though, that it would cast one better than the other, so I'd try both on
it and see which I thought it handled better.

That said, some rods are more adaptable to a range of line weights than
others are. Personal preference and casting style also play a role. I
might think a given 4/5 rod casts best with a DT4; another person might
prefer a WF4 on it; a third person might cast it best with WF5.

JR