secondary market refers to trading of securities after they have been issued. the initial issue is the primary market. e.g. an IPO is a primary issue wherein new shares are issued to the public. subsequent trading between investors in those shares on an exchange represents the secondary market.
the third market also is a secondary market, except that trading takes place away from the exchanges through electronic brokers.
so the its almost the same the main difference is that is away from the exchange?? and with respect to the fourth market is that investors don't use brokers???