It depends on the action of the 4wt. If it's a fast action it might work and since it's an older rod it might be stiff enough for crappie. I have an 8.5' 6wt. St.Croix SCV fly rod blank that I had custom made into a spinning rod and it's perfect for crappie fishing. Casts a float and jig a long way or vertical jigging brush with no problem. Try the fly rod and see how it works, use a 1/32 to start with a small spinning reel.