Hi,
I set up a server with four hard drives and software raid 10 a month or two ago. Although I can't say it's been pushed very hard yet, it's been stable so far. I did get a kernel panic at one point while setting it up, but I was also running a patched VMware server on an XFS filsystem, and since both of those are also new to me I can't automatically blame the software RAID. Performance-wise I haven't run any benchmarks, but it seems 'fast', subjectively speaking
My biggest issue was deciding how to partition the disks and which partitions to use to create the RAID 10 array. I wanted to be able to have any single disk (or possibly two) fail and still have a fully functional and bootable system. With hardware RAID, this is no big deal since the RAID is totally transparent to software.
The problem with software RAID 10 is that no boot loader can read off a software RAID 10 array! All grub or lilo sees is four drives with raid 10 data on them, which is as good as garbage to a boot loader. In order to achieve the reliability I wanted, it was necessary to create a small partition at the start of each hard drive. I used all four of them to create a single RAID 1 array, and installed GRUB into the MBR of each hard drive. The remaining four large partitions made up the RAID 10 array. The RAID 1 device is mounted under /boot, so any kernel upgrades are mirrored on all drives. The boot loader can then read the kernel off any of the four mirrored partitions. This setup also required that I created a custom initrd with the raid10 module included, since the kernel then has to mount the RAID 10 array as its root filesystem.
No distribution I know of has an installer that supports a RAID 10 root partition, so this all has to be done by hand and can be a little tricky. By the way, Fedora Core 6 is the OS I installed.
I would recommend you use a fifth extra hard drive with a working linux installation that you can boot off. This gives you a good place to configure the other drives from. I was in the unfortunate position of doing all the above with only those four hard drives and a cdrom drive to play with! It is doable though, so if you'd like a more detailed description of how that's done, just ask.
Good luck!