I have a friend with a 2004 4runner who had this problem. Looks to be a relay under the hood. If you open the relay/fuse box under the hood, there is a relay for the A/C...magnetic clutch relay, I think. The relay will be listed on the cover of the fuse panel.The toyota/Denso part number is 90987-02022 and is a known defective part. The replacement is# is 90987-02028. Toyota charges $60 or more for this. Advance Auto Parts part number R6034 is the replacement and can be purchased for about $14. The horn, and foglights use the same relay, so you should be able to swap them around and see if the problem is corrected. There is a very helpful thread about this at
Toyota - 4Runner . org Forum - powered by vBulletin that you may want to search. hope this helps