I tried to set zoom in and zoom out to buttons four and five on my mouse. The result was a crash, followed by crashes whenever I tried to enter a world. Wound up reverting via the options.txt.

Also, this appears: http://pastebin.com/fHS9LRtp

EDIT: or I can just, ya know, upload the right log...

I think I should've made it clearer. I'm trying to map zoom to the fourth and fifth mouse buttons, which have nothing to do with the number keys. You know how left, right and middle click are mouse buttons one, two and three? Mine has two additional keys on the side, which Minecraft knows as 'button 4' and 'button 5'.

And yes, can reproduce with Forge, Stellar API and Photoptics.

Does those keys supported with other mods? I doubt that the keys will work, since they often does not exist.

Most mods I have tried support them, yes. Even vanilla does. The only mod I've used that caused issues with mouse buttons was Psi, and that has since been fixed.

Is there any key duplication? Since 4 and 5 are already reserved by hotbar slots.
Also, can't reproduce with Photoptics only.
Are you sure that this crash always happens?