My database is running in 8i with oracle forms 4.5. So now i am going to migrate my database 8i to 11g. The question is whether Oracle11g will support forms 4.5 or 5.0? Or do we need to migrate forms version too ?
It is possible but I also recomend the same thing that Christin recomended for you. Its good habit to stay upto date with new technology. May be initially it feels lot of burden to your company but you feel happy with in short period with your decesion to mov to 11g Forms.
Well, the fact that forms 4.5 is desupported for some time should be reason enough. I guess there is no certification information available anymore for forms 4.5; but if memory serves even 10.1.2.0.2 is not certified with Database 11.2.x (you'll have to upgrade to 10.1.2.3), so I guess you can derive that all lower versions aren't certified as well.
See here for certification informations on Forms 10: http://www.oracle.com/technetwork/middleware/ias/downloads/as-certification-r2-101202-095871.html
additional certification informations (for newer versions for example) can be found here: http://www.oracle.com/technetwork/developer-tools/forms/overview/index.html