can i put a screen form the 4th gen ipod into a 2nd gen ipod

No.

They are very differenrt screens. The 4G screen has many more pixels than the screens for the previous iPods. The driver circuits on the earlier iPods will not know what to do.

Then you will esentually have a 4G iPod. However, thjer are other differences like the headphone jack is seperate on the 4G. See the following to see the difference inside the iPods:

iPod Touch Repair – iFixit

Really, just get a 4G iPod. Much easier. Or if you don't want to, why not just get a 2nd gen screen, which is made specifically for 2nd gen?

Kid, you don't have a clue. I suggest you take some electronics and computer technology courses.