Call her out. If she can publicly proclaim she only has three grandkids, she can be publicly reminded of a fourth.

So make a reply of “you forgot this grandson” and shame her. Don’t let her get away with it

While this is obviously an oversight or snub (I’m betting oversight) and hurtful to OP; it’s possible granny might be having memory problems, like dementia or it might have been the only photo she has of 3 of her 4 grands together… Unfortunately as people age, sometimes serious memory problems start or get worse. If granny is otherwise showing signs of dementia then it’s a painful situation but not necessarily an intentional snub. My own mother sometimes calls me by my brother’s name when I am talking to her and occasionally confuses facts - And she’s not showing major signs of dementia (yet)…