You can use someone else's disc 1 with your CD-key if you know someone else with the game. However, the game won't run without an actual disc 1 in the drive (since you'll need to return it to whoever you borrow it from). There are a few things you can do about this:
1. Purchase and install an expansion to the game. That expansion's disc will become the new play disc.
2. Download a no-CD/DVD crack for the game from
http://www.gamecopyworld.com. This will allow you to play once you return the disc.
Also, since you're only missing disc 1, if you can find a disc to borrow, copy it to your hard drive. These files can then be used to install the game again in the future, should you need to. Start the installation with the files from the hard drive, then use the discs you do have (2, 3, & 4) in the drive. (I know this method works, because I have to do this in order to install my non-English edition in English. It involves editing one of the files once the disc is copied to the hard drive.)
Other than that, there isn't really any 'legal' way around it other than to purchase a new set of original discs.
As for installing it on a Mac, there is a Windows and Mac version of the game, sold separately.