I prefer Fallout 4, and by quite a lot.
Fallout has a superior cast of characters, a better story, better writing, a better settlement system, more diverse locations, better atmosphere, better environmental storytelling. better weapons/armor and a better crafting system, and the world itself was absolutely packed with unique places to explore and things to find. Despite being limited to a caricaturized version of a single city, I found exploration in Fallout 4 vastly superior to running in straight lines for hours on every planet I landed on in Starfield. Bethesda's formula over the years has been that every time you find something of interest, you can turn in place and spot another point of interest withing a short distance, it's how people get so sidetracked in their games. The fact that each location is a handcrafted, unique place made exploration rewarding. In Starfield you can find the exact same building with identical "lore" note spread about in dozens of places in a single playthrough. You can find Fallout versions of real-world locations around Boston and dig up their stories. Hell, even the bar from Cheers in Fallout 4. At every turn there is an example of a type of humor that I only ever find in Bethesda's games, where Starfield just feels devoid of any semblance of character. Mod tools were available comparatively fast. I spent more time in Fallout 4 than any other game, well over 4000 hours, thanks in large part to mods. Though the gunplay itself in Starfield feels better, there is no VATS eqivalent, no dismemberment, hell the enemies don't even actually show any damage at all, nor do the creatures in the big empty worlds they just stop animating and fall in a heap. For all its faults Fallout 4 is a much more complete experience than Starfield was.
Starfield was my most anticipated game, ever. I've wanted a game like it since I was a kid and being a huge fan of Bethesda's sandboxes over the years, I was even more excited for their take on space exploration. In the end it was the most disappointing game I've ever played. The tedious exploration frankly sucks, there are no land vehicles, no in-atmosphere flight, everything is so limiting. The game's aesthetic is completely hit-or-miss. Ship, weapon and suit designs were awesome for the most part, but the worlds and characters themselves seem like they were created under multiple directors and lack cohesion. Massive expanses full of copy/paste buildings that you aren't the first to discover. In a game where you're supposed to be exploring the unknown, it sucks that every single place you visit has already been found by someone else, usually with their perfectly pristine corpse there. The characters in the game are all single-note personalities, many are outright unlikable. Despite being "the least buggy Bethesda game", the game runs as bad as Fallout 4 did at launch, often much worse. This game has more quest-breaking bugs in it than any Bethesda game I've played. I had to use console commands to complete five quests for a variety of issues, ship bay doors stopped opening after about 20 hours and ships in space no longer spawn NPCs outside the important unkillable NPCs but they are all non-responsive. My settlements disappeared entirely from the map and can no longer be accessed so I lost dozens of hours of work. I can't start a new game plus because the other me became unresponsive the end of the game so if I want to experience the NG+ stuff I have to restart my playthrough. I was also looking forward to getting in on the ground floor with mods and making some of my own with what I learned modding Fallout 4 but Bethesda still hasn't released mod tools and at this point I've lost interest in the game. I put a lot of hours into the game but the characters and world in the game were entirely forgettable. What a shame.