just now i have purchased iphone 4s but am unable use personal hotspot. As it is not showing the option personal hotspot.

Your carrier needs to support this feature, and such needs to be provisioned on your account in order for that setting to appear. So, contact your carrier & see if they support Personal Hotspot, & if they do, that such is properly provisioned on your account.

- settings

- general

- cellular

- personal hotspot ON

Note that carriers require a more expensive feature enabled

AT&T requires the 5GB personal hotspot plan for $50 a month along with your current voice and text packages.

Again, you won't have that setting if your carrier does not support Personal Hospot & such is not properly provisioned on your account.