Sentence:  The purpose of the metal is to speed up the <WSD>precharge</WSD>, and the reason for only 4 taps is to spread the precharge out over 15 ns to avoid a huge current spike during <WSD>precharge</WSD>. 
Meaning of the word marked with <WSD> tags:  The phase in the access cycle of DRAM during which the storage capacitors are charged to the appropriate value.