Can I replace mother board of 4th nano with a 2nd?

I do not think that this is possible due to different sizes as well as changes in connections and logic board layout. Unless of course you are really good in modifying cases as well as logic boards. Sorry, but good luck. You may just find what you are looking for on eBay etc. The Battery and the hold button should be repairable and that would give you two iPods and lots of experience. Let us know what you are going to do .

Unfortunately, this is not possible. The two units are completely different, inside and out. You would have to literally rebuild the board to make it fit. Sorry.

They are both fixable individually!

You just need to solder a new hold ribbon to the 4th gen and a new battery to the 2nd gen.

If you don't feel capable of doing it i can do it for you.