A weight forward flyline is one of the most common and versatile flylines made today. If you are trout fishing or casting short distances a WF or DT would be fine, the advantage of the DT is you can flip it around when your front part wears out, HOWEVER if you are casting long distances so the head + running line if any of the 2nd taper is also being cast then the cast will become very inefficient.
Each rod wt should be matched with its line wt. So a 6wt rod should use a 6wt line. If you were to use your 6wt reel and line on your 4wt you would overload the rod considerably. With todays graphite technology you wouldn't destroy the rod like you would if it was bamboo, but it is not a good idea. If you were to underline your rod (meaning using your 4wt reel / line on your 6wt) you would have serious difficulty loading the rod. The best idea would be to get 2 seperate reels and lines. Orivs makes very nice and fairly affordable trout reels (battenkill series etc). Linewise the best bet would be Scientific Angles (SA) or if you would prefer somthing a little less pricey go with Cortland and in my opinion go with the WF line.
Hope this helps
