So as I was trying to install DP4 last night I noticed that it will lag at a certain point, then a yellow arrow would appear stating that I should reboot and retry the installation. I tried repairing my hard disk using the Disk Utility within the Applications\Utilities folder assuming it would fix it but the same error occured when I tried to install Lion afterwards. Instead, I popped in my SL installation disk and booted up from it and used the Disk Utility from there. Tried reinstalling Lion and it worked perfectly. Not sure if anyone has tried this method, because they're so many threads on the forum talking about it. Give it a shot and let me know if it works for you as well.