There is nothing wrong about a four weight being one's main stick. If one understands what will be delivered (as in line and fly), the presentation he wants to achieve (dry, nymph, streamer), the size of the fish that he intends to target (shaker, average, Homie
), and the conditions that he will be fishing in (big water, small water, breezy, not breezy), the four weight could be the perfect tool.
Sometimes the rod choice is a regional one.
My best friend, Brian, and I are separated by 110 miles. He fishes a four weight mostly. I fish a five weight mostly. He fishes a floating line with a wide assortment of flies (dries, nymphs, small streamers). I fish a floating line, sinking lines, and sink tips weighing up to 175 grains with a wider assortment of flies (same as Brian's but including big bunnies and deer hair). Since he throws a lot of dries, he needs a softer presenting rod. Since I throw a variety of rigs, a rod with a wide presentation range is important. On Brian's waters, he has few shots at Homie. On my waters, I have more shots at Homie. Brian's waters are smaller and wind protected. Mine are larger with less wind protection.
Is the four weight losing favor? I don't think so, but in my region, the five weight has ruled for awhile.