Sentence:  One moment , the road was filled with disciplined troops , marching <WSD>four</WSD> by four with a purpose as implacable as death ; the next , a cloud of gun smoke covered a screaming fury of sound , out of which the redcoat soldiers emerged with their bayonets and their cursing fury . 
Meaning of the word marked with <WSD> tags:  the cardinal number that is the sum of three and one