Z75pro3 for overclock

Depends on many things. Not all chips are made the same so you will have to test it yourself! Also 4.5ghz might be pushing it on a heatsink, ivybridge runs MUCH hotter.

How do you know that the motherboard should not be further increased due to the multiplier?
if i don't get bsod,and not don't too hot(about 85celsius)?
automatic continous reduction and increase (from 48 to 35 and 35 to 48) of the multiplier indicates that the motherboard reached the maximum limit?
because on 47x-48x my multiplier continous changed automatically from 48 to 35 and 35 to 48 if i running prime95

and one another question
somebody told me that i should left the turbo value on default.
and if i want to overclok i need raise the cpu voltage offset.
what is the difference?
because i think both value helps raise the Vcore.but what is the difference?what do you recommend?