Sonic X ended with its third season in 2006. While there have been some rumours and fan wishes for a continuation, there are no official plans or announcements from Sega or the creators for a fourth season.
The third season concluded the main story arc of Sonic X, which saw the heroes returning to their own dimensions after a long adventure.
It's worth noting that the series has seen some "continuation" in the form of the Sonic X manga and Sonic X comic which featured additional stories and characters. However, these are separate from the television show.