I've had a bunch of Pi Zero W cameras in 24x7 use for years in a very harsh desert environment where ambient air temps regularly exceed 120F, and have never had a single crash of any of them, not even a failed SD card despite being written to daily for storing motion-activated images (the sunrise/sunset is always captured). I don't even have fans on these, and they're in the compact pi foundation red and white case for camera use. It's really been quite impressive.

What I've heard is a crashing Pi is usually due to sagging voltage on an inadequate power supply, and that's probably even more an issue on the more demanding/powerful Pi4. On my cameras I just use the wall warts adafruit sells for the Pi, and haven't had any power problems.

As long as you use something like the official PSU and don't connect particularly power-hungry USB devices to it you should be fine.

As far as the SD card goes you can boot off an external hard drive or SSD via USB and that should guarantee longevity.

My biggest problem with headless Pis was that they would randomly disconnect from the wifi network and disappear, turning off power saving on wifi didn't solve it. In the end the real solution ended up being to just create a 2.4ghz-only network and connect them to that. Somehow they just don't play well with networks that have the same SSID for 2.4 and 5ghz on my Ubiquiti gear.

Be careful with the gear you use on your Pis, just like any system. If you’re using a no-name USB-C power supply and dirt cheap generic SDHC cards, you will likely have problems. By default, the Pi4 doesn’t come with a power supply and while you might be tempted to power it from your PC, spend the money for the official power brick, or, even better, get the official Pi Power over Ethernet hats. For SD, buy from a reputable source and get a brand name (Samsung or Sandisk).

This is anecdata from my part, but I’ve got eight or 10 Pis running (Flightaware/ADSBexchange, PiHole/Influx/Grafana/Wireguard, a couple of Octoprint, Home Assistant - with a relay HAT for triggering dampers, Showmewebcam, and a handful of other things). Once I switched to good power supplies and good SDHC cards, my problems went away. My most heavily loaded machine is the PiHole machine which also has influx running and writing to the SDHC card - it’s probably a ticking time bomb, but it’ll celebrate it’s fourth birthday on the same card in two months.

What power supply were you using?

My Pi 4 running 2 instances of Octoprint kept crashing until I switched to their official power supply. On previous models I always got away with whatever power supply I had around but the 4 doesn't play around when it comes to power consumption.

If it was one of the original Pi4s you would need to get the FW update that fixed the USB chip power that was the source of many overheating issues -- though that came out relatively soon after release.

Not if you get a good SD card, or a USB SSD.

you can disable all logging in pihole - which i do and my sd card from 2016 is still going strong.