Fly lines wts are a very foggy business. Some lines like the cortland 444 and H&H lines fall within AFFTA line wt standards, others exceed the AFFTA line standard by a half or more. Furthermore if a caster carries a lot of line in the air when casting the grain wt of the total head length might be 1-3 line wts higher than what the line is rated by the manufacturer. A 5wt line might actually be a 7wt line when 45 ft of line is beyond the rod tip. This is especially true with double taper lines because the have no running line.

If you really want to get geeky about the line wt stuff Umpqua sells a line wt scale for around $20. It can be a real eye opener when comparing line wts. from different manufacturers.

So if you like a 4wt rod with a 5wt line then by all means use the 5wt line. Line wt designations vary so wildly from manufacturer to manufacturer that there really is no industry standard anymore.