How long does an iPad 4th gen last for not battery I mean in Years

No. Once sn iOS version comes out that is not compatible with your hardware, you cannot, obviously, update iOS any longer. However, your iPad running the last release of iOS compatible with it will continue to run that iOS release at the same speed it always did.

Nothing about any computer ever slows down with age - your cpu, RAM and storage all function at the same speed as they always did. New software may add features that tax your system more than a newer system would be taxed, but nothing about your system changes just because the device ages.

At some point, our system simply is unable to update or run newer software because that newer or updated software requires hardware features that yours simply does not have. But your exisiting software on your existing hardware will continue to run just as it always has.

As to when your system becomse unable to handle new software or iOS features, that is purely a matter of how quickly newer hardware comes out and how quickly software developers move to new features that your system cannot handle.

There is no inherent reason to believe that an iPad 4 will not be compatible with the next release of iOS - it might not be, but it may very well be. Depends entirely on what changes the next release (whenever that may be) introduces, and what new device hardware Apple may be working on.

No, there is no reason to believe the iPad will become slower when a newer version of Software is released. My iPad 2 has now had version 4, 5, 6, and now iOS 7. There is no noticeable speed difference. It will continue to operate on iOS 7 as long as the battery hold out.

As long is software is available you should be able to give it to your grand chidren.

One definition of "taxing" is attempting a task that requires a great deal of effort or energy.