In the early days, we tried to run 4 tasks at a time on both RPi3 and RPi4 (up to 4GB). We experienced reboots, apparent freezes, web interface not accessible - but the RPi apparently still running. So the Balena people experimented with us to reduce the numbers and it was found that for 1GB systems, one task at a time worked reliably. 4GB systems ran reliably with 3 tasks.
The snag was that the 1GB systems were put into their own queue for “small” tasks that they could run. This quite often ran out of work, but if your one is running 1 task then that’s fine. I transferred my 4 x RPi3 and one of my RPi4 to another project which has the work divided up in smaller chunks.
I still have 5 x RPi4 (4 x 4GB + 1 x 8GB) running on here.