Despite Fallout 3 bringing the Fallout franchise back and bigger then it ever dreamed of being Fallout New Vegas barely mentioned Fallout 3 beyond a handful of off beat remarks and ED-E's character which was irrelevant to the events of New Vegas beyond the reason as to why ED-E made the journey west.

they barely mentioned New Vegas on Fallout 4. There's some very very hidden ref here and there, but I think they avoided purposefully.

Why would they mention the events of New Vegas? Only six years had passed and the distance between Vegas and Boston is over 2500 miles. It seems hard enough just for the Brotherhood who do have the ability to contact their West Coast branches to stay in contact never mind random wastelanders with their own home grown problems like the Institute and their machinations never mind all the other everyday issues from living in the Wasteland.

Its not like they act like the West Coast doesn't exist. Kellogg is from the NCR (which hints at his age and spoils the 'twist' when you find Shaun), Nick Valentine has reports of the Mysterious Stranger operating in the NCR, Deacon jokes about House, Jack Cabot's ambition to go explore the Mojave Desert in search of alien shit, Vera Keyes is referenced during Far Harbor by a pre-war actress Robo-brain that was jealous of her success, Nuka World makes references to Sunset Sarsaparilla since it stalled Nuka Cola's expansion in the west.

If they had referenced New Vegas directly you'd probably be complaining about why it was being brought up since it'd be irrelevant to Fallout 4, the Commonwealth and the story in general since it likely would 'ruin' things in the process.