hello guys 

can i know the difference between 4AX and  6AX addresses 

Hello fsoliman355,
as fritz already said, these names indicate the size and communication direction of the allocated i/o area. 

In case your question is directed to a generic understanding of this topic, then I have nothing to add to the comment fritz gave already.

In case your question is directed to the detection at runtime, then you are asking for the size and type of the slot of that device in question on that particular PROFIBUS address an this DP Master system. In general terms. You want do typify a geographical (Masterssystem, Rack = SlaveID, Slot, SubSlot) address. If that is what you are seeking, then you should look into some system functions of the mastersystem (your S7-400 system).
And, Yes, that is possible to know the difference.