Some of the sytems(like cisco) use 4 layers for tcp/ip but some of them use 5 layers (Application, transport, network, datalink, physical). It totally depends upon the protocol developed by the system.
@sdtechi - regarding your comment about rubberman. First, you come across as being very rude and arrogant. Second, I guarantee you that rubberman has a lot more knowledge about networking that you do and I don't even know him personally.
If you understood that these models are purely conceptual, you wouldn't be asking these questions. The number of layers isn't really that important. The idea behind the models are to illustrate where and how protocols are related to one another in the TCP/IP communication process. The layers also provide a way to organize similar protocols that perform a similar service.