Q: How can we recognize between iPad 3 and iPad 4

What do you mean exactly? If your asking how to tell witch one is diffrent check the charging port. The iPad 3 uses the old 30-Pin charging port and the iPad 4 uses the Lighting Connector witch is very thin and small

You can also check the model number for your iPad, this can be found on the back at the bottom-middle of the device and will usually start with an A.  Further info is available here: http://support.apple.com/kb/HT5452

But in a nutshell what Drake136 says is spot on, if it's the small connector at the bottom of the iPad then it's a 4th gen for sure.

Regards,

Steve