Q: We're sorry, we are unable to continue with your activation at this time.I have I Phone 4 and its not getting activated. Please help to get it fix asap.

This message usually means the activation servers have detected an iPhone that has been hacked and /or a computer previously used to hack an iPhone

Assume you have just obtained this iPhone ??

Is this what you are seeing?

If you can't activate your iPhone, iPad, or iPod touch after installing iOS 9.3 - Apple Support

I have I Phone 4

The linked article states:

Updating some iOS devices (iPhone 5s and earlier and iPad Air and earlier) to iOS 9.3 can require entering the Apple ID and password used to set up the device in order to complete the software update.