is it a must that my laptop have a copy of the apps.. i have 4 computer, seems like all 4 have a copy damn.

You can download any apps thru Wifi or 3G to your iPad, but not a must you need a copy on your iTunes / computer, you need a copy on your iTunes because of someday maybe you delete certain app, but another day you want it back to your iPad, so you can easily sync it back to your iPad, but no need download thru Wifi or 3G ( because M'sia broadband is SLOW ). Or unlcuky maybe someday your iPad firmware crash / corropted, after restore a new firmware, you no need 1 by 1 download to your iPad.

If you want all ur purchase app have a copy in your computer, you can plug your iPad to your computer, from the iPad icon in iTunes, you can right click then choose " transfer purchase ", so all your purchase app will keep a copy in your iTunes.
*** You might need to authorize your Apple ID in your iTunes if you can't transfer purchase, just in iTunes select " Store " > " Authorize This Computer ".

Notes;
You may authorize maximum 5 computer or laptop under one Apple Account ID. If you find you have reached 5 authorizations, you can reset your authorization count.

Remember to deauthorize your computer before you sell it, give it away, or get your computer serviced.

To deauthorize a computer;
1.Open iTunes.
2.Choose Store > Deauthorize Computer (In earlier versions of iTunes, access this option from the Advanced menu).
3.Select "Deauthorize Computer for Apple Account" and enter your Apple ID and password.

If deauthorise then that will release the number of comp that I authorised right? ie. if already authorise 5 pc, deauthorise one then will be able to authorise the other pc?

Stil dun quite understand how we can share apps between devices (e.g. with my friend)

Not too sure how to share with friend (different Apple ID).

But, Apple has a 'rolling out' a feature that you don't need to sync app for paid app that you purchased on different device, this is on the same Apple ID.

Tried, try to search the app you have on the different iOS, then it will appear as install instead of the price of the app.