The color temp usually doesn't mean much in terms of plant growth, both of those bulbs will grow plants.

Now whether you like the color is a different matter. The 4100k is a cool white and will have a yellowish tint to it (compared to higher k bulbs). The 18000k has a violet tint to it. If you like the colors, keep them. If you think the 4100k is too yellow try a 6500k or above bulb.