I still have iPhone 4s. What is the best IOS version for my iPhone?

There are only two things you can do for your iPhone 4s. Either use the current iOS that is on the device, or use iOS 9.3.3, the current iOS. You cannot "find" or "install" any other versions of iOS.

This article may help in upgrading to iOS 9.3.3

Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support

is there any way to stop the pop up message displaying the new update ... since we don't want to update the IOS? to keep it with the existing IOS for example ...

Yes but....

Go to Settings > General > Storage & iCloud Usage > Manage Storage (the first one) > and if there is an update, delete it.

Then restart the phone.

The problem with this is that the next time you charge the phone while connected to Wi-Fi the update will redownload again and you will get the pop up again and also the storage loss. So the only way to prevent it (other than blocking at the router) is to disconnect from Wi-Fi while charging.

I wont to update my ios