One serving against a day's fiber
The Dietary Reference Intakes set an Adequate Intake for total fiber of 25 g a day for women aged 19 to 50 and 38 g for men of the same ages. Measured against those, the 4.3 g in a 1 piece/slice, any size serving of cake, fruit cake is 17% of the first and 11% of the second. US food labels use a single Daily Value of 28 g instead, which puts this serving at 15%.
Calorie for calorie, the figure is 1.1 g of fiber per 100 kcal (this serving is 373 kcal). Among the 110 packaged with USDA energy data here that is 36th, and 698th across all 957. The intakes above were derived from 14 g of fiber per 1,000 kcal, or 1.4 g per 100 kcal, so this food sits below the ratio a whole day of eating is meant to reach.
Getting all 25 g from cake, fruit cake alone would take 6 servings, and 38 g would take 9. Most days reach the target through several different sources.
Soluble vs Insoluble Fiber: What Each One Actually Does · All fiber guides