英語wikiより
Zombies spawn in groups of 4 and will pursue the player on sight from 40 blocks away, as opposed to 16 blocks in other hostile mobs.
ってことで現状は40ブロックだそうな
ただしこれは1.6.1の変更で
Zombies are now able to detect the player up to 40 blocks.
これ以上広くなったってこと？
