What is the purpose of using these two models collection.
why can't we use resource model for all collections.
Why we are using MySql4 collection for simple module
Some one explain.
I would say in magento 1.8 all collections are resource collections.
Resource collections depend on generic implementations for all the SQL-stuff. the idea is to only reimplement the base class to offer support for another sql database, like postgre, oracle or mssql.
The still existing Mysql4 classes are only for backwards compatibility but only extend the resource collection
I'm not sure about this, but I would say you are maybe talking about Models (extending