i tend to agree with D2A. i have never had dura reduce failing from +4 to +5 before. but i am not a player who does +5 alot which is why i came here for confirmation.
anyway after seeing the feedback here, i added steady to my +4 boots and tried +5. weee! it succeeded! then i added steady to my +4 chest and tried again. it failed

but the steady was still there. i guess there is a chance to get dura reduce on +4->+5 but the chance is slim.