Apart from a locked bootloader, it appears Samsung is now trying to stop their latest S4 from being rooted.

Over on XDA Developers, user AndreiLux discovered the following –

“I just noticed when sniffing the new 9500UBUAMDK kernel is that they activated a new “feature” / CONFIG_SEC_RESTRICT_SETUID in the kernel.

This means no more rooting is possible, at least not anymore in the usual methods. Your SuperUser will tell you it will have granted access, but it’s neutered on the kernel level.”

For those that like rooting, the S4 may not be the best option. Read the full thread here.