The current SVG file for the 人名用漢字 version of 牙 (07259-Jinmei.svg) contains 4 strokes and is pretty visually similar to the standard version. This seems to be a mistake.
There might be an official 人名用漢字 document somewhere that specifies the exact stroke count for each character, but I could not find it after a brief search. However, the extra stroke is mentioned in the entry for きば【牙】 in 明鏡国語辞典 ("Meikyo") and also on the
Japanese version of Wiktionary.
Meikyo: 『常用漢字表では、一画多い「
〓」も同字とする。』
Wiktionary: 『
新字体: 4画、朝鮮: 5画』
The Wiktionary page will display the Jinmei version of the character correctly only if you have a Korean font available in your browser. For reference, here is an image attachment for both forms of the character (standard and Jinmei, respectively) in Adobe's Source Han Sans font.