Why the hard drive can only have 4 primary partition. If i want do more, what should i do?

Create an extended partition. Use one of those four primary partition to create four extended partitions. In that way you can have partition 1,2 and 3 for use as primary partitions. Partition 4 would be an "link" to extended partitions 5,6,7 and 8 which will be for your usage.

thank you for your reply i will try it when i have the opportunity

Let me guess, this is a commodity i?86 based system? Put it down to classic PC architecture historical limitations.

You can have an extended partition which allows four more contained partitions. This is a good example of extending a standard while trying to avoid breaking it.