hi, you can:
1. get access to any computer with a dvd-reader.
2. From there you build a disk image (depending on the computer and operating system there are various softwares to do that. This should be rather simple).
3. Then you copy that disk image (which is a single file) from that computer to an USB key
4. You plug the USB key to your MAC
5. You install LR4 from the USB key, seen as a disk (DVD) by the MAC
Hope this will work for you