Thats right folks, the GH3 DLC will in fact work in GH4. Now, obviously since GH3 DLC only uses the guitar,  only guitar parts will be playable. No word on whether an updated version is expected to be released but it sounds unlikely right now.

Luckily it seems that going forward all new GH installments and their DLC should be fully compatible with the new instruments and be useable without any problems.  This includes the upcoming Guitar Hero: Metallica which is set to support the full complement of instruments and will support all of the new DLC as well as the GH3 DLC.

The caviat to all of this is of course that GH: Aerosmith DLC will not be supported, so a word to the wise would be to avoid GH: Aerosmith DLC if not the game itself.

Sources are a Kotaku Article and info found on the Official GH Forums.

Awesome BUT I dont even own any of the previous once so it not a factor to me.
Im buying GH:WT for Wii and fuck RB for that shovelware shit they gave the Wii.

I dont see a reason to buy RB 2. I guess if you like the set list....I dont.

It doesn't effect you right away but it does mean that going forward DLC will stay with you and you won't have to switch discs/pay multiple times/etc... to utilize it.  At the very least it is a good precedent being set as we go forward. 

I'm personally a lot more likely to buy DLC now than I was before...but I really really hate the idea of DLC to begin with so I'm in the minority from what I can tell.

All very true.

And like you DLC is meh to me. Id only buy it if its a band I really like and none of the music games really have any of the bands I like.