这四个词都可表示“工厂”，注意以下用法：

【1】factory 最普通用词，可泛指任何制造产品的地方,泛指一般意义的“工厂”，是最普通，用得最广泛的一个词。

a clothing factory 服装厂 a glass factory 玻璃厂

a food factory 食品厂 a paper factory 造纸厂

He owned furniture factories in New York State.

他在纽约州有几家家具厂。

The demand generated by one factory required the construction of another.  

一家工厂产生的需求使得必须再建造一家工厂。

The girl had found out about the goings-on in the factory.  

这个女孩已经发现了工厂里的勾当。

The factory may make as few as 1,500 cars this year.  

今年，工厂生产的小轿车可能只有1,500辆。

【2】mill 原义指"粮食"“磨坊”加工厂,现多指轻工业方面的工厂。 

a cotton mill 棉纱厂 a silk mill 丝织厂

a flour mill 面粉厂

a steel mill.钢铁厂

a textile mill.纺织厂

His father worked shifts in a steel mill.  

他父亲是一家炼钢厂的轮班工人。

This old mill has been grinding away for over 50 years.  

50多年来,这间老磨坊一直在磨粮食.

The rice mill was wrecked by the enemy bombing.  

碾米厂遭到了敌机的轰炸.

He was assigned to the heat - treatment shop of the mill.  

他被分到厂里的热处理车间.

He wrote a meritorious theme about his visit to the cotton mill.  

他写了一篇关于参观棉纺织厂的有价值的论文.

The mill incorporated with others.  

这家工厂与别家合并了.

【3】plant 多指电工，电力或机器制造业方面的工厂。

a machinery plant 机械厂 a car plant 汽车厂

a radio plant 无线电厂 a printing plant 印刷厂

在美国英语中也泛指一般意义的工厂。

a coffee plant 咖啡厂

Ford's British car assembly plants.

福特公司在英国的汽车装配厂

The plant provides forty per cent of the country's electricity.

全国总电量的40%是这家发电厂提供的。

【4】 works 多指钢铁等重工业方面的工厂，也指一些特定的厂。

an iron and steel works 钢铁工厂 a machine works 机械工厂

a printing works 印刷厂

The steel works could be seen for miles.

这个钢厂几英里外都能看见。

a recycling works.

回收工厂

the works canteen.

工厂餐厅

备注：以上区分只是相对的，并不是绝对，也有很多通用的情况，有时可能四者都可用。

a paper mill (factory, plant, works)造纸厂

a steel works (mill, plant, factory) 炼钢厂

当然有区别，factory是大的厂(制造业)，works（机加工厂）是工作间，车间，小作坊等。

请看字典的定义：

1.factory：A building where machines are used to make things is usually called a factory.
例句：I work in a cheese factory.

2.works:A place where things are made or where an industrial process takes place can also be called a works. A works can consist of several buildings and may include outdoor equipment and machinery.

例句: There used to be an iron works here.