You can make four primary partitions in your hard disk no matter whatever may be the size of your hard disk. In your system you have one partition table or metadata file. The size of this table is 64 bytes. So whenever you create one partition, it takes 16 bytes to store its metadata in partition table. That is why you can create maximum four primary partitions.
But there is a way we can create more than four partition. That concept is called extended partition. Under this extended partition we can create logical partitions.
Hope this will give you some idea about partitions.