四字母词根: frag，fract

frag，fract破，折

fragile

fragility

refract

refractive

refractory

四字母词根: frig

frig冷

frigid

Frigidzone

frigidity

refrigerate

(refrigerator）

四字母词根: grad

grad = step, go, grade步，走，级

grade

gradual

graduate

retrograde