I have around 4 SiteCollections in a web application in Sharepoint 2007, I wonder if one site collection is for customer A, and second is for Customer B, will they be assessed using same IIS website address and is it a good practice, and will we be sharing anything between site collections.
Depends on what you mean by sharing, they will be sharing the same website in IIS, the same web.config, the same content database by default, the same web application settings in Central Admin etc etc.
You can configure site collections to be under different host headers if you wish. What are your requirements? Would you prefer each customer to be accessing a different URL? What about SSL acess, should they all be secure or just some of them.
Need some more detail really.