I highly doubt we will see any characters from the West appear in FO 4. This also greatly depends on the date the game takes place. If it is 2277, you can automatically rule out anyone appearing from FONV, unless there was an ED-E pre west coast journey, or Whitley(IIRC), the scientist who created ED-E and is a mentioned only character.
The date also can determine if we see anyone from FO 3, or if there are any time skips. If the game is Oct 2277, it highly unlikely we see anyone from FO 3 unless there is a time jump, then it could be possible to see someone.
If the date is sometime after FO 3, like a 2278+, then it could be highly likely we see someone from FO3, in particular Madison Li, especially seeing how she bolts for the Commonwealth pre-Broken Steel, sometime during the 2 weeks the LW is unconscious.
Anyone appearing from FO1 is super highly unlikely, as they would more than likely be dead, unless they were a ghoul or super mutant.
FO2 it COULD be possible, as 35 years have passed, except for Marcus who we know will appear in NV, and more than likely Cassidy would be ruled out, as would Myron(dead). I would rule out Vic also, as he doesn't appear to be the type to travel 3000 miles, and if he did anything would probably go to New Arroyo with "boss".
So, FO 2 companions that could possibly still be alive: Goris, Sulik, Robodog, Lenny, K-9, Brain Bot.
I would imagine after FO2 Sulik and the CO and crew would probably go looking to rescue Sulik's sister. After that, maybe he would travel. Depends on the mood of Grampy Bone maybe. Goris could be alive and travel, Lenny who knows, and the rest are robots of some kind so that also a big who knows.
FOT: Highly unlikely, unless the BoS in FO4 are the MW BoS, then maybe a General Barnaky cyborg or robot of some kind, otherwise, extremely unlikely.
FO3 it all depends on the timeline, any time skips, etc. If it is the CW BoS in FO 4, then we could get various recurring characters from the BoS, maybe Reilly's Rangers, Madison Li is an obvious choice, maybe Zimmer, Harkness, Talon Company Mercs, scattered Enclave remnants, the treasure seeker woman, Sydney(IIRC) seems like a roamer type, maybe a Regulator presence. It really depends on the timeline. The closer FO 4 is to FO 3 timeline, the less likely there will be recurring characters from FO 3, but if it is 10 years later, it could be just about anyone.