is it ipad 1 1st generation - 4th generation or are they completly different as in ipad 1 then ipad 2 then ipad 3 then ipad 4

The original or "1st generation" iPad is often referred to as the iPad 1.

The second update to the iPad is referred to as the iPad 2, the 3rd update as the iPad 3 and the fourth update as the iPad 4.

Some people will refer to each update as a "generation" of the iPad, but really what it amounts to is a release of an updated model (an updated model means a new step in generation). Sometimes, the update is substantial (e.g. the iPad 2 to the iPad 3) and other times less so (e.g. The iPad 3 to the iPad 4).

See Apple's guide to iPad models - http://support.apple.com/kb/HT5452

Technically they are all different models - each generation has at least some hardware differences.

Every "generation" of every Apple device has a unique model number.