Which application server I should use while testing and while implementing in some small companies which application server best describes their needs.
Does the client enforce the use of some particular appserver (as a company standard)? If not, then select the one you're most comfortable with (assuming licence costs are not an issue). If the system needs to interact/co-exist with other J2EE applications, it's often easier to use the same application server as the others (simplifies environment management).
Should I use Java application server provided by Sun along with J2EE 1.4 beta release. In what way it is different from iPlanet or other app servers.
The Sun J2EE 1.4 Beta is a
reference implementation and
beta. If you're comfortable with that, then fine. I would choose something else.