知之为知之,不知为不知,是知也。
the four 知 mean "know"
之: it (something)
为: be (is)
不: negative
是: this / that ------ attention, 是 doesn't mean "be" in old chinese.
the last 知 = 智 (u can google: 通假字), it means 智慧
也: interjection
so this phrase means:
knowing something is knowing something, not knowing something is not knowing something, this (is------this verb is omitted) wisdom.
an english-style phrase:
Say yes if you know, say no if you do not know, which is the wise behaviour.