Thanks for all the reponses that I got for this thread, each one is really
helping me to understand this temp tablespace.

I did some research and find out that dropping and creating the temp tablespace
will not effect the database. So I did that.
I did create temp tablespace and assigned 50 MB of space to it, temporary type
with Pctincrease = 0, datafile name as temp0.dbf. It took me about
one hour to drop the old 4G of temp tablespace. Creating that temp
tablespace was quick. Everything seems to ok. I checked the tablespace and datafile size from Oracle
enterprize manager. It looks the same as I was creating for.

But when I looked at size of the temp tablespace datafile
at the physical location under the oracle datafile folder.
I was amazed to see that I am having new temp0.dbf file about 3.8G
space along with the old one of 4G of temp tablespace. I deleted the 4G of temp
tablespace to free the space.
But I could not understand why this new temp file has that much of space?
And what causing to have that much of space for this temp file?
And why I am not seeing that space occupying in DBA Studio?

Any further thoughts.

SKM