I'm afraid... we have bad news...
As many of you know, Flash Sentry is a new character introduced in 'Equestria Girls' the movie. As sad as this may seem, some script writers for 'My Little Pony Friendship is Magic' have declared that Flash isn't coming up in Season 4.
We can't say for sure that he isn't coming back, so we'll do a bit of research. Hang on tight!
-Mlp Max