Difference between 3rd and 4th market?

Anyone can explain the difference better than the books? Thanks.

The only difference what I understand between third and Fourth market is that “Fourth market uses specialized electronic network instead of normal communication method for trades between Institutional investor” Fourth and Third market both are for the Institutional clients (Non exchange registered participant) who trades (OTC) between each other on Exchange listed securities, to save on the brokerage fees. The other benifit is that it does not impact the security prices in the secondary market.

The difference is that you either trade with a counterparty directly or through a ECN which is considered to be a new market. Nobody in real life uses the distinction, though. In several textbooks you will only see three markets.