名詞 fourth:

1. following the third position; number four in a countable series
2. one of four equal parts
   同義詞：one-fourth, quarter, fourth part, twenty-five percent, quartern
3. the musical interval between one note and another four notes away from it

形容詞 fourth:

1. coming next after the third and just before the fifth in position or time or degree or magnitude
   同義詞：4th, quaternary

副詞 fourth:

1. in the fourth place
   同義詞：fourthly

1. I can't do the fourth and fifth questions but I've done all the others.
   第四題和第五題我不會，但是剩下的我都答了。
2. After her fourth child she decided to be/have herself sterilized.
   她生了第四個孩子後決定做絕育手術。
3. Mars is the fourth planet in order from the sun.
   火星是太陽系的第四顆行星。
4. Time is sometimes called the fourth dimension.
   時間有時被稱為第四度空間。
5. One fourth of a yard; nine inches.
   四分之一碼; 九英寸
6. I am a soldier of the fourth cavalry.
   我是第四騎兵部隊的一員。
7. One fourth of a ton;500 pounds. Used as a measure of grain.
   五百磅四分之一噸; 五百磅。用作谷物計量單位
8. After her fourth child she decided to have herself sterilized.
   她生了第四個孩子后決定做絕育手術。

fourth

n.（名詞）
The ordinal number matching the number four in a series.
第四：一序數(shù)詞，一列數(shù)中的第四個
One of four equal parts.
四分之一
Music
【音樂】
A tone four degrees above or below a given tone in a diatonic scale.
第四音級：自然音階中高于或低于一給出音調四度的音調
The interval between two such tones.
四度音程：兩音調之間的間歇
The harmonic combination of these tones.
四度和音：音調的和諧組合
The subdominant in a scale.
下屬音
The transmission gear or gear ratio used to produce forward speeds next higher to those of third in a motor vehicle.
第四檔：海車換擋時從第三擋換到下一擋，以加快速度
Fourth The Fourth of July; Independence Day.
Fourth 美獨立紀念日：7月4日；美國獨立紀念日

來源：
Middle English fourthe
中古英語 fourthe
from Old English fôorth3 * see k wetwer-
源自 古英語 fôorth3 *參見 k wetwer-

【引伸】
fourth
adv.（副詞）
adj.（形容詞）