Fourth light in a group of lights not lighting up

I’ve searched for the issue, but could not find any solutions to the problem so I’ve decided to post a thread for it.

I’m trying to make a light that doesn’t self illuminate, look like it is. For this I’ve surrounded it with four lights:

http://puu.sh/kxmdj/c583decb21.jpg

However, when rendering only** three **lights lit up, leaving the fourth one dark:

http://images.akamai.steamusercontent.com/ugc/392168636451364925/EA8D87649BA7EB2A3554E655B98C8CC628675124/

Does anyone have any idea why the compiler is not compiling the last fourth light?!

Thanks for any help.