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Use your Time Machine, or other backup/cloning program you have been using, to restore your Mac to the older version of OS X that was originally on your system.
Or you might need to set the Write permissions for your user name on the folder that LR uses to store the license info in.
Go to the folder described in the article above and right click on the folder and select Get Info. At the bottom, Sharing and Permission, your username should be listed and have both Read & Write permissions. If not click the Lock icon, type in your password, Click the Plus sign and add your username then give it the Read & Write permissions.
With El Capitan Apple has gotten more restrictive with user permissions.
Just a friendly note.
The next time you have a deadline for a project don't upgrade your operating system in the middle of it. Wait until you are done with that project and have free time.
Then make sure you have a total, complete, system backup from just before you did the upgrade that you can restore in case something goes wrong.
just a friendly note it made me update because it said lightroom wouldn't work and i did have a total complete system backup that i payed a professional to do that does it for a living but he said he doesn't work with lightroom so he didn't know why it was not working after i put the serial # in but thank you for your help! I'm not stupid about updating i make sure i have my *** covered just wasn't sure how to make the serial # work and why it wasn't plus i had my stuff for the deadline done but not saved on the computer because when he went to do everything for me i was at my other job!
Please check the permissions of the folder LR stores it registration file in (the folder and file serial number is stored in).
I used a Mac several years ago and still have virtual machines of OS X running on a Windows system and I have never been forced to do a OS X upgrade.
LR 4 came out in March of 2012 and the last update for it was released in February 2013. At that time OS X was at 10.7 Lion then went to Mt Lion in July of 2012.
As far as I know LR 4 would work on versions of OS X from Snow Leopard to Mt Lion.
Since you have now upgrade to El Capitan you might need to upgrade LR to either the CC version or the stand alone version 6.
But from what I read other people have been using LR 4 on El Capitan. You have a permissions problem.
Look at this thread.