Even circles are linear. A circle of finite size comprises a finite number of dots, each of which is connected to its neighboring points by a line segment. Thus, a circle is nothing but a bunch of microscopic lines dithered together and which appear to be curved at sufficient distance.
So yes, Fallout, and all RPGs are linear. All games are linear, everything in the Universe is linear. Nothing in existence is nonlinear except waves, which are actually also particles, which can be connected to form line segments, which are linear.
. . . seriously though . . .
There is no such thing as a "non-linear" game. Even Minecraft is linear. Everything you ever do in a game can be reduced to a "sequence of events" and even if those events at a distance are obviously "going in circles" they are still connected points in a succession.
I think the real question that you are interested in is: "Is Fallout 4 'Open-World'" and/or "Does playing Fallout 4 facilitate 'sandbox play.'" Another slight variant of the actual question at hand would be: "For the game overall, are there various ways to play it; are they all reasonably promising prospects in terms of 'fun potential' (to an average player), and does the game facilitate all of them to a roughly equivalent degree or are some disporportionately leveraged onto the player?"
I. Open-World? Absolutely. Once you get out of the vault you go in any direction you want. Not as gigantic a world as say a Minecraft world, but with far, FAR more detail and atmosphere.
II. Sandbox play 'friendly?' Most definitely. This is precisely what the game is, with the oft-criticized Settlements, as well as crafting and just plain exploring being exemplary of this.
III. Comparable facilitation of varying playstyles? Yes and No. Trying playing the game as a pacifistic demagogue with high Charisma . . . probably a lot more challenging than playing as a gun-wielding badass. The "Wasteland" being portrayed favors a few specific playstyles over others and I would place them in the following order of precedence:
A. Use of weapons and/or violence to overcome obstacles;
B. Use of mobility and cover to mitigate or avoid damage;
C. Use of sneaking to achieve tactical advantage (though I find that completely "sneaking past" would-be enemies is far less possible in this Fallout than in many past and similar games);
D. Use of lock-picking to break into places and containers;
E. Use of charisma to convince others;
F. Use of hacking to break into computers.
G. Use of cover to evade damage;
H. Use of damage resistance to tank damage;
I. Use of chems, medicines, and perks to circumvent or remediate damage.
This is perhaps the games most serious flaw: you can "finish it" by focusing inordinately on being a gun-wielding, run-and-gun, super-trooper: sneak only a bit, basically ignore all locked doors and containers (couple exceptions I think, but very few), never once attempt to make a charisma check in dialogues, never once hack into a computer (again, there are probably one or two exceptions but very few overall), generally eschew cover or tactical ingenuity, even ignore power armor and only gather up the chems you find in the abundant containers in the dungeons you will clear and you can probably "finish" most of the game. I put "finish" in quotes because I do not actually consider it to be "finishing" but you can basically 'get to the cut-scenes' that tell you "You finished that quest/quest line" which is what a lot of gamers seem to think "finishing" a game is really all about.
If you want to be an "old-school RPG" player, you can, and the game doesn't even punish you for that. But you do note HAVE TO. This is the big difference with the game compared to say FO2, or other older-style RPG and character development games (Temple of Elemental Evil; Baldur's Gate; Jagged Alliance, etc.).