In the magazines Dragon and/or Dungeon during the fourth edition of D&D there were occasionally articles on various demons in a series of articles called the Demonomicon of Iggwilv (not to be confused with the rules supplement published as a book called Demonomicon). For instance, in Dragon issue 364 there was an article on Yeenoghu. I vaguely remember one of the issues having a similar article about Graz'zt but I can not remember which issue, or even which of the two magazines, it was in.
Yes, in Dragon #360, but it was for 3.5 not 4e. There was an article from the 4e era in Dragon #414, but it wasn't a Demonomicon of Iggwilv article
It was in Dragon magazine #360 pg 9. That was right before 4th edition came out.
The article is more crunch than fluff, but there is some cool plot hooks in there, especially detailing his capture of Waukeen during the Time of Troubles.
Crunch wise, it has a CR 32 stat write up of the G-man (from the 3.5 rule set); a prestige class, a new monster, and some new metamagic.
You may be thinking of the article in Dragon 414 entitled History Check: The Iggwilv-Graz’zt Affair By John “Ross” Rossomangno. It is 4th ed, but is almost all fluff, save some history check DCs.
Perhaps the greatest love story in D&D lore is the on-again, off-again affair between a demon prince named Graz’zt and the witch-queen Iggwilv.
For a basic history and biblography on Graz'zt, there is a neat article written by Shannon Appelcline on the Wizards web site.