As most fans of the series know, FE4 is divided in two generations, the first of which ends on a tragic note with all the characters either dying, getting captured, escaping, or with their whereabouts unknown. The fate of several characters remains unknown even after you finish FE4 and play FE5, which is a mid-quel to FE4.
There is, however, an interesting scene in FE5 that may tell us what happened to a few of these characters. Check this image:
http://i.imgur.com/7HTCt8W.png
This takes place after you beat Chapter 4. The guys in red sprites are all bad guys, and they're in a secret room that went unnoticed by the good guys (this is from the map itself, the secret room is covered by a roof while you normally play the map).
What's interesting about this scene isn't the villains, though, but look at the four statues in the room. One of the villains in here, Veld, has the power to turn people into stone, and as it will be established later in the game, sometimes they turn their enemies to stone and keep them as trophies. This seems to imply these statues are indeed captured enemy soldiers, or, Sigurd's troops from FE4 Gen 1.
The statue on the top left is a female Swordmaster. The only one who fits this is Aira, so it's most likely her, as we never know what happened to her in FE4.
The one on the top right is a dismounted male Mage Knight. The only one that fits this is Azel. Given he is Alvis' young brother (Alvis is one of the main villains in FE4), it makes sense he wouldn't be killed, but they stoned him for whatever reason.
The bottom left one is rather odd. It's a dismounted female Mage Knight, but the sprite fits on Tiltyu (Mage Fighter) and Lachesis (Master Knight). It can't be Tiltyu, since FE4 establishes she dies between gens (Hilda tortured her so much she died of depression), so I guess Lachesis fits. An interview with a designer said that there was supposed to be a map in FE5 where several captured units have to be un-turned to stone, one of them supposedly being Lachesis, so that seems to support this theory.
The bottom right one is hard to figure out since it's a dismounted male cavalier, so a lot of characters fit. It could be Alec, Noish, Lex, or Beowulf.