Good news - I just set my 2GB Pi4 up and left it, and just looked at it now and its running a job. There are several not started for lack of memory, but I guess it will attack another one of them when the current one finishes.

In your Account, Computinmg preferences,

Memory
When computer is in use, use at most     95 %
When computer is not in use, use at most 95 %

will give BOINC more RAM and should allow 2 Tasks to run at a time.

In

Other
           Store at least 1    days of work
Store up to an additional 0.02 days of work

would be good options to avoid ending up with more work than you can process before the deadlines while the Estimated completion times settle down.

Memory
When computer is in use, use at most     100 %
When computer is not in use, use at most 100 %

Other
           Store at least 0.1    days of work
Store up to an additional 0.5 days of work

In one of the earlier threads in this forum.
Rosetta running on ARM platforms

Perfect thanks - I now have my spare 4G Pi running a number of tasks. Sadly the 2GB Pi 4 just hasn't quite got enough memory and the 3+ at 1GB definitely doesn't

That sounds like it didn’t take the alt_platform tag. What BOINC version and please list your cc_config.xml

<!--
This is a minimal configuration file cc_config.xml of the BOINC core client.
For a complete list of all available options and logging flags and their
meaning see: https://boinc.berkeley.edu/wiki/client_configuration
-->
<cc_config>
  <log_flags>
    <task>1</task>
    <file_xfer>1</file_xfer>
    <sched_ops>1</sched_ops>
  </log_flags>
  <options>
    <alt_platform>aarch64-unknown-linux-gnu</alt_platform>
  </options>
</cc_config>

Then please sign up for World Community Grid on the smaller device.
They've got COVID-19 tasks coming soon for Windows, Mac and Linux and need their queues drained of dry of everything else ASAP.
Also their system requirements are usually much smaller.

In one of the earlier threads in this forum.
Rosetta running on ARM platforms