Why the official requirement? Just because you don't want the device to be abandoned? Or is there some other "official" value-add I'm unaware of?
I'm going to ignore the "official" requirement because I think your task might actually be impossible with that req.
I use an Xperia XZ1 Compact with LineageOS 20.
4.6" screen. Very similar in dimensions to the iPhone 5/5S, which had 4" screens (just a tiny bit winder, maybe a mm taller). Headphone jack. microSD slot with support up to 512GB. Side fingerprint sensor. Decent (not modern flagship quality, but also not an AI "image" generator) camera. Band 66 support (though no band 71). And a notification LED!
Sony only supported it with two software updates (big surprise) ending with Android 9 (Pie). The Sony website lets you unlock the bootloader with a self-serve portal. You can use the Xperifirm tool to flash stock software & firmware, and custom roms work as usual with flashing, TWRP, etc. The SD card makes custom ROMs an even simpler process.
Note that you need the US firmware for cellular band & network compatibility, but you can copy the two files related to fingerprint sensor usage (system_X-FLASH-ALL-C93B.sin
and vendor_X-FLASH-ALL-C93B.sin
) from CE1 (chinese) or UK firmware into the US firmware, flash it, and you'll get US firmware with a functional fingerprint sensor.
There's recent builds of Lineage 17,18,19, and 20 (unofficial, unfortunately) with relatively few compromises. Gapps versions and MicroG versions seem to get published once every month or two by the unofficial maintainers. Yes, they're unofficial. But they've been publishing new builds every month since 2019. And allegedly the only reason they haven't gone official is because Lineage puts some restrictions on packages you can include in official releases.
There's also HavocOS and a relatively recent /e/OS build, if you're into that.
Unfortunately my mobile provider (Google Fi) has some compatibility issues with these ROMs, but there seem to be dozens-to-hundreds of happy users out there. The modem has a tendency to crash when signal is completely lost, so if you live in the middle of nowhere like I do, I don't think I would recommend it. But if you live in a city or a country with competent infrastructure you should be fine.
The call microphone has a shitty physical design; it uses sound piped into a single, very long, very tiny hole in the bottom of the phone, next to the USB-C port. Guess what? Over time that tends to fill with dust, and then people can only hear you on calls if you hold the phone at the perfect angle, shout, or switch to speakerphone, which uses a different mic. Fortunately you can clean it pretty easily with a SIM card ejector.
TL;DR this is literally the last reasonably sized phone by a major manufacturer you can use as an actual smartphone with custom ROMs. And there's a good amount of custom ROM support out there.
The other one I would seriously look at is the Pixel 4a, but it's definitely above your size requirement. It might be more useful if you list a height limit or width limit to the phone size instead of a screen size limit -- for instance, I won't use anything taller than 135mm. But the iPhone SE (2016), XZ1 Compact, and iPhone 12/13 Mini all satisfy that requirement, despite having 4", 4.6", and 5.4" screens, respectively. Aspect ratios and bezels are weird!
I'm definitely a small phone lover. I don't watch videos on my phone if I can avoid it and tend to consume text-heavy content.
Currently bouncing between an Xperia XZ1 Compact running Lineage 17.1 and an iPhone SE 2016 (!!!) running iOS 15. I get security updates for both, but it's clear that I'll have to move to a different phone in the next year or two.
Honestly at this point I just wish Apple would bring out an "iPhone Classic" based on the 4,5, or Mini body. Throw a headphone jack on there, a TouchID power button, and I'd pay serious money for it on launch day. I'd prefer an SD card slot, but if I know the phone will last for 3-5 years, I can pay a couple hundred extra bucks for 512GB of storage.
I just do not get folding phones. I understand that others like them, but I've never been a tablet guy. I suppose I would consider one with an internal e-ink screen, so I could combine my e-reader and phone into a single device... but I don't think I'd enjoy using a phone that's 1.5-2x the standard thickness of modern phones (not counting those massive camera bumps!). Plus the durability issue -- I would absolutely not trust any modern folding phone to last 5+ years, and at the $1000+ price point, it damn well better last 5+ years.
Above all else, I prefer small phones for two reasons:
It's really frustrating that phone companies don't bother with small phones any more. But nobody is even trying -- and never has tried -- to market a small phone that stays out of your way but helps you when you need it. Even Apple barely marketed the Mini at all (and debuted it during a global pandemic when a lot of people stopped commuting and traveling, two of the best times to have a small one-handable phone).
I wonder how much Apple fucked up the small phone market by maintaining exclusive access to the Mini screens from Samsung, as the author mentions in this article.