Funny..If you own the game & have infact installed it with your game you can use the inbuilt program to make an installer than can be spread over what ever disk size you choose..My Steam L4D install is 4.7GIG but when using steam to back it up..( right click game in main steam window) & select backup game files...Then choose DVD5 & it will compress it to a single installer that is only 2.4gig in size..
So to recap the only way this will not work is if you DONT own the game..
EXTRA info..It compresses at a ratio of about 50% as ive backed up Counter Strike Source & L4D & it still fit to a single disk..
But for backing up purpose I would suggest you Reinstall steam somewhere new & then re download L4D or install it & that way you can back up just that game. This can be done even if you bought the CD version as its the key that activates the game not the disk..