Sorry, but I'll have to disagree with flytyer here. I pretty much exclusively use a DT line on my 4 wt rod for trout, salmon, smallmouth, and anything else in a river. There are several advantages to a DT line, one being a softer, more delicate presentation. If you're casting a fly to spooky trout, a WF line makes a bigger plop in the water than a DT line.
A second reason to use a DT line is that it may match better with your rod action. A soft action rod may load up too much with a WF line, especially when trying to make longer casts. I speak from experience on this one. I use a very soft action rod and it doesn't cast nearly as well with a WF as it does with the DT line. On long casts, the WF line overloads the rod and I can't cast nearly as far as I can with the DT line. So a WF line won't always cast farther.
My passion for catching fish is eclipsed only by the fish's passion not to be caught.