I have had an original S3 with a 1TB HDD since 2008. It has never experienced a random reboot. I got an S4 XL4 this past February and recently it seems that it has a random reboot about once a week or so. I'm not sure if it's related to the TiVo Stream or not.
Should I be concerned? This is the one electronic device ever that I've opted for a warranty on, and I do have the 3 year service warranty on my S4.