I have a Raspberry Pi 4 8GB. Never happens to me that situation, only a few errors where the result file with a different size than expected

My second RPi experiences a similar problem with another OPN WU - OPN1_0052146_00119 - see below:

OpenPandemics - COVID 19 7.17
State	Running
Received	2021-06-01 10:50:57
Report deadline	2021-06-08 10:50:57
Estimated app speed	0.46 GFLOPs/sec
Estimated task size	31'832 GFLOPs
CPU time at last checkpoint	20:10:48
CPU time	20:30:54
Elapsed time	20:43:43 
Estimated time remaining	36d,09:01:56
Fraction done	2.319% 
Virtual memory size	152.48 MB
Working set size	92.93 MB

The other concurrently running WUs look fine.
After 1 year, experiencing the same problem on two machines within less than 2 days is maybe not a coincidence
Cheers,
Yves
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Current kernel: 5.10.17 / Raspios

Yves - no Pi 4 here just 1x 3A+ and 2 x 3B+. They are housed in transparent cases from eBay with tiny 30 mm fans - enough to keep the CPUs cool enough and almost silent. 40 mm fans are much noisier.

Asked about what power supply you were using as I tried using PC and modem USB ports, but could see the red light in the case occasionally blinking and decided that might not be good for reliable results with the CPUs at 100% - so am using the official 5.1 volt Pi supplies.

Two of these Pis are used for DNS and Pihole - and the third will become my webserver. Also have a Zero WH for testing and experimentation - but is too slow for WCG. The 3B Pis boot from SD card which just has the /boot partition - so is only is use for a very short time. Swap /root etc are all on the SSD (very inexpensive 120GB WD Green SATA SSD).

The 3B cases - https://www.ebay.com/itm/362856676049

I have a RPI3 running OPN WUs, with a zram swap, and occasionally it gets very close to filling both ram and swap (i know because i have a simple monitoring setup). Most WUs seem to use 80-160M ram, but some WUs use up to twice that, and that's when it starts to swap.

Reading your comment, you have swap on SD card. Do you have also an in-memory swap (either zram or zswap) ? IMO it's a bad idea to have swap on SD card without zram/zswap in between. Increasing the swap on the SD card might help somewhat, but i'd avoid it based on my previous experience.

as for SD card in general (not swap usage): in my experience, if you tweak your OS a little, to minimize writes, it works fine. I've been running boinc on 6 linux SBCs (2 RPIs, 4 ODROIDs) for ~7 months, nonstop, full load (1 WU per each core) and except for one card with problems, all have been stable.