Q: Is it safe to update iphone 4 to iOS 7.06

Depends what you mean by Safe.

If the Device has been Modified / Hacked / Jailbroken... Then No.

If this is the case... Sorry... But...

The discussion of Jailbroken Devices is against the Terms of Use of this Forum.

Unauthorized modification of iOS  >  http://support.apple.com/kb/HT3743

Only if you want to turn it into a brick.

IOS 7.0.6 bring major fix seecurity  in your phone as it is fix SSL/TLS failed. so you should install it to be safe

Every person I know with an Iphone 4 only wants one thing and that is IOS 6 back. I cant even hang up a call. And yes I have done all your fixes. Phone was fast before IOS 7 period.

as I read this will be fix in IOS 7.1