Is there a way to split a hard drive into more than 4 primary partitions? I am installing multiple OSs, and it would make life a lot easier if there was a way to do this. My hard drive is 1TB.
You can use GUID partition table instead of MBR partition table. GPT can handle 128 partitions by default, but you can increase that number by resizing partition table size, as stated in this article posted in comments by @Vality.
Note that GPT is relatively new and older OSes may be unable to handle it correctly. In some cases your GPT can be destroyed completely if software doesn't support it.
A grub menu booting 100+ systems of Dos, Windows, Linux, BSD and Solaris (forum thread) can be helpful if you want to stick to MBR.
Yes, this is possible even with an MBR partition table. There's a special partition type, Extended Partition. It's essentially another partition scheme enclosed in an MBR partition. Wikipedia has a surprisingly detailed description of this scheme.
So long as the boot loader resists on a regular partition, there are no relevant limitations. Still, some operating systems may throw a tantrum. :)