Hi shahidul1,
When you say you, "removed a phone but now have it back" it sounds like it was lost but recovered. Before it was lost it had a 4 digit passcode on it, but now is is asking for six digits.
No, there is no default passcode on the iPhone. It is setup by the user.
If you are using Two-factor authentication however it will want a six-digit code.
Two-factor authentication for Apple ID - Apple Support
iOS 9 also defaults to a six-digit passcode.
See Use a passcode with your iPhone, iPad, or iPod touch - Apple Support
If you can't use your passcode, please see this article
If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support
If it's an issue with activation lock, then see
Find My iPhone Activation Lock - Apple Support
Best regards.