Well that takes into consideration with what you think are part of the series. Having personally not played Fallout 4 I can't comment on it.
Depends on what you seek. For a fan of the series it is a blashpemy.
If you want a top down, engrossing RPG experience, you want FO1 and 2.
If you want a good shooter with admittedly rusty gunplay, but a good progression system and a decent storyline, pick up FO3.
If you want a version of FO3 with even more freedom to change the plot and infulence the world around you, great modding potential, AND better gunplay/companions, however a slightly worse story, pick up NV.
If you want a great shooter that has tons of gear, options to customize said gear, possibly the coolest power armor in any fallout, and a massive amount of freedom, with a risky, revolutionary perk based, unlimited progression system, pick up FO4.
I, personally, am a fan of it. Is it classical Fallout? Not in any way, shape, or form.
But it's a great game, easily GOTY.
From what I've seen, anyway, I can't run it.