Here is the example.
First Let me say what are the entities contributes
to 5 tier  Arch.
1.Client Tier - Browser , Applet , client
application
2.Presentation Tier - Servlets,JSP
3.Business Tier - EJB or some business objects.
4.Integration Tier - JDBC ,
5.Backend Tier - Data bases,JMS etc.

Hope you might have understand the 5 tier
architecture.

Tier 3/4 are same (?).In EJB DataSource to Access Database related
stuff.

Maybe change:

3.Business Tier - EJB or some business objects.
4.Integration Tier - JDBC

to:

3.Messaging Tier - JMS
4.Business Tier - EJB or some business objects.

We cannot change this...
JMS will be in the Back end tier.
If you use JMS the 3 and 4 becomes single tier  and
it will use MDB for integrating with JMS.
So what I stated is correct.
The 4th tier cannot be exist without 3.
This what in the J2EE specification.
Me too confused first.Now I am pretty clear about
that.
We use call the same as 3 tier earlier.Now they
split it according to logical tier where these 5 tiers
are loosely coupled.
Hope this explanation will clear your doubt
mr.srinivas.

Ok, but I'm gonna refute that and say that JDBC is not a tier  it's a
specification for developing drivers for communicating with the
database/persistence tier. Or by JDBC do you mean an overall data
access layer so your original list would look like:

1.Client Tier - Browser, Applet, Client Applications
2.Presentation Tier - Servlets, JSP
3.Business Tier - Business Objects (Session EJB, Business Classes)
4.Integration Tier - Data Access Layer (Entity EJB, JDO, SQL)
5.Backend Tier - Databases, JMS etc.

The 3 and 4 are same.Now the 3 tier  is split  into 5
tier according to its logical entity .If you look into
these 5 tiers they are very much loosely coupled one.