Gee…where to start?
Here are some WAGs. Apologies if any of this presumptuous regarding your knowledge. @DrewXT just beat me to the punch regarding some items.
I’ve yet to load HA OS, but if it were me, I’d load the Terminal add-on, get comfortable with the CLI commands, and go through the logs to see if anything appears out of the ordinary.
If it’s new enough, the Rpi4 has a boot diagnostic feature worth exploring:
Beyond that, I’d suspect power, the SD card, and perhaps misconfiguration.
Rpi-4 power requires a 5.1 V supply with a current capacity of at least 3 A. If you’re using USB power for a power-hungry peripheral, consider a powered hub to isolate the current demand of the peripherals from the Rpi.
SD cards have a wear-out mechanism limiting the number of write cycles before functional failure. A sizable
home-assistant_v2.db file might be a predictor of running out of cycles. Among other possible root causes, an increase in the frequency of crashes could be an indicator of SD wear-out.
As an aside, the rating of the SD card is generally associated with speed and I’ve seen Class 10 or U1 (or better) being recommended. Some devices include some amount of “wear leveling”, which distributes write operations across the entire memory. This technique can maximize the life of an SD card (generally with a speed penalty), but the more sophisticated strategies are used on SSDs. Terms like “surveillance” or “high endurance” might suggest the presence of the feature, but there’s no standard terminology for the technique. Some devices could be optimized for sequential addressing and others could be better suited for random operations. Caveat emptor.
If after looking at the above and you come up without any smoking gun, I’d make a full backup, move it offline, and reload the HA OS from a fresh SD card. Better yet, you might consider an external USB SSD, but there’s a potential drawback (RFI). After that, I’d consider restoring the configuration a bit at a time to isolate crashes to a particular integration or add-on.