There weren't really the "Yonko" yet, but Oda initially planned 4 villains for the Grand Line (Shanks was already introduced, but he wasn't among the hypothetical "Yonko" apparently, while Blackbeard was already in Oda's plans as an extremely powerful villain... considering this, he would be probably the final villain).
Weekly Shonen Jump #1557 - No. 37-38, 1999 Issue. (Teaser for the 4 villains of the Grand Line.)