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.